²¡Ô­ÉúÎïѧʵ¼ù½Ìѧ-ʵÑé½Ì²Ä:ҽѧ΢ÉúÎïѧʵÑéÖ¸µ¼(×Ô±à)Ó¢Óï¶ÁÎMedical Microbiology

Ó¢Óï¶ÁÎMedical Microbiology

ÕÆÎÕºÃÒ»ÃÅÍâÓ¶àÒ»°Ñ´ò¿ªÒ½Ñ§¿Æѧ֮ÃŵÄÔ¿³×!

At glance at any newspaper confirms the central role of microbiology inall

of ou

r lives.Will there be a vaccine for AIDS? Can genetically engineeredbacteria be

 put to work cleaning up oilspills or making anticancer agents? Can we continue

 to develop antibioticsfaster than bacteria can evolve with resistance to them?

Will other scourges follow smallpox into the history books,leaving thehuman rac

e finally free of major epidemicsª²or will new killer diseases appear totheir p

lace?¡¼¡½

vaccine:ÒßÃç

AIDS£º°¬×̲¡

antibiotics:¿¹ÉúËØ

smallpox£ºÌ컨

epidemics:Á÷Ðв¡

Such questions demonstrate that today microbiology remains one of themost r

elev

ant of the sciences.Microbiology has long been intimately involved withmatters

of life and death,and this is no less true in the age of AIDS than itwas more t

han a century ago,when Pasteur was developing the first modernvaccines.But micr

obiology today is also on the cutting edge of scientific advance,as itwas in th

e seventeenth century when Leeuwenhock was making his simple microscopesand gli

mpsing the microbial world for the first time.Genetic engineering,forexample,wa

s born in the laboratories of microbiologists and spent its youththere.In count

less areas,from agriculture to evolution,from ecology todentistry,microbiology

is both contributing to scientific knowledge and solving human problems.¡¼¡½

Pasteur£º°Í˹µÂ

Leeuwenhock£ºÁÐÎÄ»¢¿Ë

Chapter 1. An Overview of Microbiology

After completing this chapter you should be able to

¡ñDefine microorganism

¡ñList the major groups of organisms studied in medicalmicrobiology

¡ñIdentify the contributions to microbiology made byLeeuwenhoek,Paste

ur,Koch,Ehrlich and Fleming

Microbiology is the study of microorganisms,which are minute livingthings,i

ndiv

idually too small to be seen with the naked eye,and exist as singlecells or cel

l clusters.Microbial cells are thus distinct from the cells of animalsand plant

s,which are unable to live alone in nature but can exist only as parts ofmultic

elluar organisms.A single microbial cell is generally able to carry outits life

 processes of growth,energygeneration and reproduction,independently of other c

ells,either of the same kinds or of different kinds.¡¼¡½

microorganisms£ºÎ¢ÉúÎï

reproduction£º·±Ö³

Today,we casually accept the fact that microorganisms are found almosteve

ryw

here.There are more than 100,000 species of microorganisms in theearth.Based on

 the differences of theirstructures and chemical compositions,microorganisms in

 the medical microbiology areclassified into bacteria,mycoplasma,rickettsia,spi

rochete,chlamydia,actinomyces,viruses and fungi.¡¼¡½

bacteria:ϸ¾ú

virus:²¡¶¾

fungi£ºÕæ¾ú

Microorganisms are part of the human environment and are therefore important

 to

human health.For the few microbes that cause disease,learning how suchdiseases

a

re transmitted and how to diagnose,treat and prevent them is of greatimportance

 in a health sciencecareer.Such knowledge will help you care of patients and av

oid becoming infected youreslf.¡¼¡½

microbes:΢ÉúÎï

The First Observations

In 1673,the Dutch amateur microscope builder Leauwenhock was the firstto ob

serve

 and describe the ¡°animalcules¡± through his simple,singleª²lensmicroscope.Pro

gress in understanding the tiny organisms came quickly only in thenineteenth ce

ntury when improved microscopes became available and wildlydistributed.Better m

i

croscopes have enable scientists to penetrate ever deeped into themysteries of

t

he cell.Several workers discovered ways to stain microorganisms withdyes to mak

e them more visible.¡¼¡½

microscope£ºÏÔ΢¾µ

stain:Ⱦɫ

Microbiology as a science didn¡ät develop until the latter part of theninet

eent

h century.This delay occurred because,in addition to microscopy,certainbasic te

chniques for the study of microorganisms needed to be devised.Investigationof t

he nature of contagious disease led to the development of thesetechniques and l

aid the foundation of microbiological science.¡¼¡½

contagious:´«È¾µÄ

Fermentation

In 1860,a group of French merchants asked Pasteur to find out why wineand b

eer

soured.They hope to develop a method that would prevent the spoiling ofthose be

verages shipped long distances.Pasteur found that microorganisms calledyeasts c

onvert the sugars to alcohol in the absence of air.This process iscalled fe

rmenta

tion and is used to make wine or beer.Souring and spoiling,which occurlater,are

 caused by a different groupof microorganisms,called bacteria.In the presence

of air,bacteria change the alcoholic beverage into sour waste productknown as a

cetic acid.¡¼¡½

yeast£º½Íĸ¾ú

fermentation£º·¢½Í

acetic acid:´×Ëá

Pasteur¡äs solution to spoilage was to heat the alcohol just enough tokill

most

 of the bacteria,this processdoes not greatly affect the flavor of wine or beer

.

We call this process Pasteurition,and it is used for milk as well assome alcoho

lic drinks to kill bacteria.¡¼¡½

Pasteurition£º°Í˹µÂ

Ïû¶¾

The Germ Theory of Disease

The realization that yeasts plays a crucial role in fermentation was thefir

st c

oncept to link a microorganisms¡ä s activity to physical and chemicalchanges in

organic materials.This discovery altered scientists to the possibilitythat micr

oorganisms might have similar relationships with plants and animalsª²specificall

y that microorganisms might invade other organisms and cause disese.Thisidea wa

s called the germ theory of disease.¡¼¡½

germ£º²¡Ô­¾ú

The first proof that bacteria actually cause disease was given by Kochin 18

76.Ko

ck discovered rodshaped bacteria now termed Bacillus anthracis in thatblood of

cattle that had died of anthrax.He cultured the bacteria on artificialmedia and

 t

hen injected samples of the culture into healthy animals.When theseanimals beca

me sick and died,Koch isolated the bacteria in their blood and comparedthem to

t

he bacteria originally isolated.He found that the two sets of bloodculturing co

ntained the same bacteria.¡¼¡½

Koch:¿ÆºÕ

Bacillus anthracis

Ì¿¾Ò¸Ë¾ú

medium.media(¸´Êý)£º

ÅàÑø»ù

culture:ÅàÑø

Koch thus established a sequence of experimental steps for directlyrelating

 a s

pecific microbe to a specific disease.These steps are actually a set ofcriteria

,known today as Koch¡äs Postulates:¡¼¡½

Postulates:¶¨Àí

1.The same pathogen must always be present in every case of the diseas

e.

2.The pathogen must be isolated from the disease host and grown in pur

e culture.

3.The pathogen from the pure culture must cause the same disease when

introduced into a healthy,susceptible laboratory animal.

4.The pathogen must again be isolated from the introduced animal and m

ust be shown to be the original organism.¡¼¡½

pathogen:²¡Ô­Ìå

host:ËÞÖ÷

pure culture£º´¿ÅàÑø

Koch¡äs postulates provided a tremendous spur for the development of thesci

ence

 of microbiology by stressingthe use of laboratory culture.In order to successf

ully study the activities of a microorganism,such as a microorganismwhich cause

s a disease,one must be pure.Without objects as small as microorganismsascertai

ning purity is not easy,for even a ve¸ß¼¶Ö°³Æ¿¼ÊÔÍøry tiny sample of blood or animalfluid may

 contain several kinds oforganisms that may all grow together in culture.¡¼¡½

Koch realized the importance of pure cultures.He developed severalingenious

 met

hods of obtaining them,of which the most useful is that involving theisolation

of single colonies.He inferred that colony had arisen from a singlebacterial ce

ll that fell on the solid nutrient medium surface.In the 20 yearsfollowing the

formulation of Koch¡äs postulates,the causal agents of a variety ofcontagious d

iseases were isolated,forexample,C.diphtheriae,V.cholera,T.bacillus,Strep.pneum

oniae.¡¼¡½

single colony£ºµ¥¾úÂä

Vaccination

In 1798,almost 70 years before Koch established that a specificmicroorganis

ms c

auses anthrax,Jenner embarked on an experiment to find a protection fromsmallpo

x.¡¼¡½

Jenner£ºÇÙÄÉ

When a young milkgirl told Jenner that she could not get smallpoxbecause sh

e al

ready had been sick from cowpox,he decided to put the girl¡äs story to atest.Fi

rst Jenner collected scrapings from cowpox blisters.Then he madeinoculations wi

th the cowpox material by scraping the patient¡äs arm with a poxinfectedneedle.

T

he scratch would turn into a raised bump,in a few days the patient wouldbecome

mildly sick and recover,and would never contract either cowpox orsmallpox again

.We now know that cowpox and smallpox are caused by viruses.Because thecowpox v

irus closely resembles the smallpox virus,it induces immunity withoutgiving hum

ans deadly infection.The process was called vaccination from the Latinword ¡°va

ca¡±for cow.Theprotection from disease provided by vaccination or by recovery f

rom the disease itself is called immunity.¡¼¡½

cowpox£ºÅ£¶»

inoculation£º½ÓÖÖ

vaccination£ºÒßÃç½ÓÖÖ

Pasteur contributed significantly to the emergence of immunity with hiswork

 on

vaccines for cholera and rabies.In 1879,when Pasteur was using chickensas his e

xperimental animals to study cholera,he accidentally used an old choleraculture

 to inoculate somechickens.The chickens did not develop disease symptoms.When h

e later inoculated the same chickens with a fresh choleraculture,they  remained

 healthy.Though he hadn¡ätplanned to use the old culture first,he did realize t

hat the chickens had been immunized against cholera.Pasteur reasonedthat the or

ganisms must have lost their ability to produce disease but retainedtheir abili

ty to produce immunity against subsequent infections by their virulentcounterpa

rts.¡¼¡½

cholera:»ôÂÒ

rabies£º¿ñÈ®²¡

Now,some vaccines are still produced from avirulent microbial stainsthat st

imul

ate immunity to the related virulent strain.Other vaccines are made fromkilled

virulent cells or from isolated components of virulent microorganisms.Researcher

s

 have recently appliedrecombinant DNA techniques to the development of vaccines

 against a variety of virusesincluding those that cause herpes,heapatitis B and

 influenza.¡¼¡½

herpes:ðåÕî

heapatitis:¸ÎÑ×

The Birth of Modern Chemotherapy:

Dreams of a ¡°MagicBullet¡±

After the relationship between microorganisms and disease wasestablished,th

e ne

xt major focus for medical microbiologists was the search for substancesthat co

uld destroy pathogenic (diseaseª²causing) microorganisms without damagingthe

infected animal or human.The treatment of disease by chemical substanceis calle

d chemotherapy.¡¼¡½

chemotherapy£º»¯Ñ§ÁÆ·¨

Ehrlich is recognized as the imaginative thinker who fired the firstshot in

 the

 chemotherapy revolution.As amedical student,Ehrlich speculated about a ¡°magic

 bullet¡± that could hunt down and destroy apathogen without harming the infect

ed host.Ehrlich coined the term chemotherapy and launched a search forsuch a bu

llet,and in 1910 he found a chemotherapeutic agent called 606,an arsenicderivat

i

ve effective against syphilis.In 1935 the sulfa drugs were  discovered by Domagk

.Extensions of Domagk¡äs work led to the development of isoniazid,aneffective a

gent against tuberculosis.¡¼¡½

syphilis:÷¶¾

sulfa:»Ç°·

isoniazid:ÒìÝÎëÂ

tuberculosis£º½áºË

In contrast to the sulfa drugs,the first antibiotic was discovered byaccide

nt.W

hen culturing bacteria,Fleming almost tossed out some culture platesthat had be

en contaminated by mold.Fortunately,he took a second look at the curiouspattern

 of growth on thecontaminated plates. There was a clear area around the mold,wh

ere the bacterial culture had stopped growing.The mold was lateridentified as P

enicilium notatum and in 1928,Fleming named its active inhibitorpenicillin.Howe

ver,purification of penicillin proved to be a very difficult task.Thegreat need

 for such a drug in world war¢ò,money from the Rochefeller Institute and the ha

rd work of pathologist Florey,biochemist Chain,accomplished thetask.Since then,

many other antibiotics have been found,such as streptomycin andtetracycline.

¡¼¡½

plate£ºÆ½°å

mold£ºÃ¹¾ú

penicillin£ºÇàùËØ

streptomycin£ºÁ´Ã¹ËØ

tetracycline£ºËÄ»·ËØ

Microbiologist,pharmacologists and clinicians are now working on avariety o

f te

chniques to target antimicrobial drugs.The idea is to attach a drugmolecule to

a soª²called bullet,which then travels like a homing device to its target,aspec

ifice site of infected cells.The bullet thus releases the drug,which ¡°attacks¡±

 without hurting nearbynormal cells.Such a method would permit the use of drugs

 now regarded as too toxicfor general use.Thus the search for a better targeted

magic bullet continues -perhaps with more success soon to come.Advancesin cell

biology and immunology have provided researchers with information aboutseveral

instruments that eventually might turn chemotherapy into a more potentmedical w

eapon.Future bullets might include the ¡°ghost¡± or membranes of red blood cells

; a group of highly specific molecules called monoclonal antibodies;immunotoxin

,monoclonal antibodies attached to toxins; and artificial fatty globulesknown a

s liposomes.¡¼¡½

antimicrobial£º¿¹Î¢ÉúÎïµÄ

monoclonal antibody£ºµ¥¿Ë¡¿¹Ìå

The inadequate targetting of drug and the lack of antiviral drugs areonly t

wo li

mitations of modern chemotherapy.An equally important problem is theemergence a

nd spread of new varieties of microorganisms that are resistant toantibiotics.

The quest to solve these problems requires sophisticated researchtechniques and

 correlated studies neverdreamed of in the days of Koch and Pasteur.But before

w

e continue our discussion of medical techniques that control microbes,weneed to

 know more about the microbesthemselves.¡¼¡½

antiviral£º¿¹²¡¶¾µÄ

Microbiology today is on the cutting edge of scientific advance.Theoverwh

elming

 influence of microorganismsin human society is clear.We have many reasons to b

e

aware of microorganisms and their activities.As Pasteur expressed it :¡°The role

 of the infinitely small isinfinitely large¡±.¡¼¡½

Chapter 2. Functional Anatomy of Bacterial Cell

After completing this chapter you should be able to

¡ñlist the steps in a Gram stain and describe theappearance of a gram

ª²positive and gramª²negative cell after each steps.

¡ñIdentify the three basic shapes of bacteria.

¡ñExplain the differences in cell wall,stainreaction,pathogencity and

 antibiotic sensitivitybetween gramª²positive and gramª²negative cells.

¡ñIdentify the functions of bacteria cell structures.

Gram Stain

Microorganisms are too small to be seen with the naked eyes,so they mustbe

obse

rved with a microscope.Modern microbiologists have access to microscopesthat pr

oduce,with great clarity,magnifications anywhere from 10 to thousands oftimes m

ore powerful than those of Leeuwenhock¡äs simple single lens.¡¼¡½

magnification£º·Å´ó±¶Êý

Because most bacteria appear almost colorless through a standard lightmicro

sco

pe,one often must prepare them for observation by fixing and stainingthem.The G

ram stain was developed in 1884 by the Danish bacteriologist Gram.It isone of t

he most useful staining procedures in medical microbiology because itdivides ba

cterium into two large groups:gramª²positive (G+) and gramª²negative(G-).¡¼

¡½

fix:¹Ì¶¨

Gram stain:¸ïÀ¼ÊÏȾɫ

In Gram staining procedure,the heatª²fixed smear is covered with crystalvio

let.

After a short time,the dye is washed off and smear is covered withiodine.When t

h

e iodine is washed off,both gramª²positive and gramª²negative appear darkviolet

.Next,the slide is washed with an ethanol.This solution is decolorizingagent,wh

i

ch removes the violet from the cells of some species but not fromothers.The alc

ohol is rinsed off and the slide is then stained with safranin,a reddye.¡¼¡½

smear:ͿƬ

crystal violet:½á¾§×Ï

iodine.£ºµâÒº

slide£ºÔز£Æ¬

The smear is washed again,boltted dry,and examinedmicroscopically.Bacteria

that

 retain violet after thealcohol has attempted to decolorize them are classified

 as gramª²positive.Bacteriathat lose the violet color after decolorization are

classified as gramª²negative.It turns the gramª²negative bacteria pink orred.

¡¼¡½

The following general features of bacteria can be observed with thelight mi

croscope:

1.Almost all bacteria have a semirigid cell wall.

2.If bacteria are motile,their motility is usually achieved  by flagel

la.

3.Bacteria are unicellular,although they are frequently found in chara

cteristic groupings,each cell carries out all the functions of theorganism.

4.Most bacteria multiply by binary fission,a process which a single ce

ll divides into two identical daughter cells.¡¼¡½

flagella:±Þë

unicellular:µ¥Ï¸°ûµÄ

binary fission:¶þ·ÖÁÑ

Size,Shape And Arrangement Of Bacteria Cells

There are a great many sizes and shapes among bacteria.Most bacteriafall wi

thi

n a range of 0.20 to 20 ¦Ìm in diameter,and have one of three basic shapes:the s

pherical coccus;the rodª²shaped bacillus;and the spiral.In addition tocharacter

istic shapes,many bacteria also are found in distinctive arrangemnts ofgroups o

f cells.¡¼¡½

coccus:Çò¾ú

spiral£ºÂÝÐξú

When cocci divide to reproduce,the cells can remain attacked to oneanother,

they

 might not separate.Coccithat remain in pairs after dividing are called diploco

c

ci;Those that divide and remain attached in chainlike patterns arestreptococci

;Those that divide at random planes and form grapelike clusters are staphylococc

i.Those groups are frequently helps in the identification of certaincocci.¡¼¡½

diplococci:Ë«Çò¾ú

streptococci:Á´Çò¾ú

staphylococci:ÆÏÌÑÇò¾ú

Structures External to the Cell Wall

In studying human anatomy,we must look carefully at the physicalarrangement

 of

organs and tissues in the body.Because all bacteria are unicellular,thestudy of

 microbial anatomy brings usto inspect a single cell¡äs construction:how each

cellular component contributes to the structure of the cell as a whole.¡¼¡½

anatomy:½âÆÊ

1.Capsule

Capsule is generally composed of polysaccharides,which are made insidethe c

ell,

excreted to the cell surface, firmly attached to the cell wall.Not allbacterial

 species producecapsules;however,capsules are an important mechanism of bacteri

al virulence.Capsules play an important role in protecting pathogenicbacteria

against phagocytosis by cells of host.If bacteria lose theircapsules,they becom

e less infectious and more vulnerable to destruction.¡¼¡½

capsule:¼ÔĤ

virulence:¶¾Á¦

phagocytosis:ÍÌÊÉ×÷ÓÃ

2.Flagella

Flagella are long filamentous appendages that propel thebacteria.Bacteria w

ith f

lagella are motile,that is,they have the ability to move on theirown.Motility c

an be seen in bacteria grown in a semisolid ¡°motility medium¡±, in which movem

ent can be seen outward from the inoculum.Motility is important becauseit allow

s microorganisms to move from place to place in order to obtainnutrition for gr

o

wth and reproduction or escape from toxious macroenvironments.Flagellaoccur on

both Gramª²positive and Gramª²negative bacteria,and their presence can beuseful

 in identification.Forexample,they are found on many species of bacilli but rar

ely on cocci.¡¼¡½

inoculum£º½ÓÖÖÎï

3.Pili

Pili are hairlike appendages attached to bacteria cells in much the sameway

s as

 are flagella.But pili areconsiderably shorter and thinner than flagella.Pili o

ccur almost exclusively on Gramª²negative bacteria and are found on onlya few G

ramª²positive organisms.¡¼¡½

Pili£º¾úë

Bacteria can have two kinds of pili:(1)short,attachment pili or (2)long,sex

 pil

i.The attachment pili can adhere to the surfaces of other cells.Piliassociated

with the bacterium,Neisseria gonorrhoeae,help the microb

e to colonize mucous mem

branes. Once colonization occurs,the bacteria are capable of causing thedisease

.The sex pili join two bacterial cells and furnish a pathway prior tothe transf

er of DNA from one cell to another.Such transfers among bacteria causeproblems

for humans because antibiotic resistance can be passed on with the DNAtransfer.

Consequently,more and more bacteria acquire resistance,and humans mustlook for

new ways to control these bacteria.¡¼¡½

Neisseria gonorrhoeae:ÁÜÇò¾ú

Cell Wall

1.Function and Composition

The major function of the cell wall is to prevent bacterial cells fromreptu

rin

g when the osmotic pressure inside is greater than that outside the cell.Clinca

lly,the cell wall is important because it is the site of action of someantibiot

ics.¡¼¡½

osmotic£ºÉø͸µÄ

The bacteria cell wall is composed of a macroª²molecular network calledpept

idog

lycan(murein).This peptidoglycan layer is biochemically unique and isnot found

in any eukaryotic cells,so that one of the most important structuralfeatures of

 the procaryotic cell is thecell wall.¡¼¡½

peptidoglycan:ëľÛÌÇ

eukaryotic:ÕæºËµÄ

procaryotic:Ô­ºËµÄ

As the name implies,there are two portions to the peptidoglycanmolecules ¡ª¡ª

a pe

ptide portion and a glycan portion.The glycan portion consists of Nª²acetylgluco

samine(NAG) and Nª²acetylmuramic acid(NAM).The NAG and NAM alternate inrows, ea

ch row forming a carbohydrate backbone.To each molecule of NAM isattached a te

trapeptide side chain.Adjacent tetraª²peptide side chains may be directlybonded

 to each other or linked by apeptide cross bridge consisting of one to five ami

no acids.The backone is the same in all bacterial species,thetetrapeptide side

chains and the peptide cross bridges vary from species to species.

¡¼¡½

tetrapeptide:ËÄëÄ

cross bridges:½»ÁªÇÅ

2.Differences Between G+ and G-Bacteria

In most gramª²positive bacteria,the cell wall consists of several layersof pept

idoglycan connected by peptide side chains and cross bridges.Thisarrangement p

rovides a very rigid framework.The layers of peptidoglycan are considerably thi

cker in gramª²positive than gramª²negative bacteria.

G- bacteria also contain peptidoglycan,but in very small amounts,thepeptidogl

yc

an is probably only a monolayer thick,thus,the cell walls of gramª²negativebact

eria are susceptible to mechanical breakage.The peptidoglycan layer issurrounde

d by an out membrane structure,which consists oflipoproteins,lipopolysacchari

de (LPS) and phospholopids,This outer layer is an important barrierbetween the

cell¡äs interior and certain substances from the environment,includingantibioti

cs

 such as penicillins,certaindyes and heavy metals.So,in general,the antibiotic

penicillins are more effective against gramª²positive bacteria thanagainst gram

ª²negative.¡¼¡½

lipopolysaccharide:Ö¬¶àÌÇ

A characteristic feature of Gramª²negative bacteria is possession ofvarious typ

es of complex macromolecular LPS.So far only one Gramª²positive,List

eria monocytogenes,has been found to contain an authentic LPS.LPS,al

so known as endotoxin,

consists of core polysaccharides,O£­polysaccharide and lipid A. O£­polysaccharid

e is called O antigen and helps to distinguish species of gram£­negativebacteri

a by immunological means.Lipid A is responsible for the toxic propertiesthat ma

ke any gramª²negative infection a potentially serious medical problem.Itcauses

fever and intravascular hemolysis.¡¼¡½

endotoxin:ÄÚ¶¾ËØ

lipid A:Ö¬ÖÊA£¬ÀàÖ¬A

The mechanism for the Gram stain is related to the structure andchemical compos

ition of the cell wall.The intact,thick peptidoglycan layers of the gramª²positi

v

e cells¡ä walls prevent the crystal violetª²iodine from leaving thecells,so the

 gramª²positive bacteriaretain the color of the crystal violet dye.The gramª²ne

gative bacteria have much thinner peptidoglycan layer,probably only amonolayer,

and all out membrane composed most of LPS.This out membrane is mostly removedb

y the ethanol rinse,and the monolayer of peptidoglycan hasdiscontinuities that

allow the crystal violetª²iodine complex to escape.They are colourlessuntil cou

nterstained with red dye,after which they appear pink.¡¼¡½

counterstain:¸´È¾

3.L Form

When the peptide cross linkage is disrupted,the cell wall is defectiveand cann

o

t adequately protect the bacterial cell against osmotic shock.It is forthe reas

on the antibiotic penicillin is effective in controlling bacteriainfections.Pen

icillin prevents the formation of crossª²linkages between thepeptides,resulting

 in the production ofdefective cell walls and the death of growing bacteria.

¡¼¡½

Clinically,the cell wall is a good target for certain antimicrobialdrugs,becaus

e the wall is made of chemicals unlike those in eucaryoticcells.Thus,chemicals

that will damage bacterial cell walls or interfere with theirsynthesis,often wi

ll not harm the cells of an animal host.¡¼¡½

synthesis:ºÏ³É

Removal of the bacteria cell wall be accomplished by hydrolysis withlysozyme or

 by blocking peptidoglycanbiosynthesis with antibiotic such as penicillin.In os

motically protective media,such treatment liberate protoplasts from gramª²positi

ve cells and spheroplasts (which retain the out membrane) fromgramnegative cell

s

.The bacterial cells with defective cell walls are called L forms (namedafter t

he Lister Institute,where they were discovered).¡¼¡½

lysozyme:Èܾúø

biosynthesis:ÉúÎïºÏ³É

protoplast:Ô­ÉúÖÊÌå

spheroplast:Ô²ÇòÌå

L forms are difficult to cultivate and usually require a medium that issolidifi

ed with agar as well as having the right osmotic strength.Some L formscan rever

t to the normal bacillary or spirillum from upon removal of the inducingstimulu

s.Thus,they are able to resist antibiotics that interfere with cell wallsynthes

is,so, L form infections are relatively special problems inchemotherapy,their r

eversion to the bacillary from can produce relapses of the overtinfection.¡¼¡½

cultivate:ÅàÑø

agar:ÇíÖ¬

bacillary:¸Ë¾ú

spirillum:ÂÝÐξú

Struetures Internal to the Cell Wall

Plasma Membrane

Cytoplasm

Nucleoid

Endospores

When essential nutrients are depleted,certain gramª²positive bacteriaform speci

alized ¡°resting¡±cells called endospores.Endosporesare highly durable,dehydrat

ed bodies with a thick wall.They are formed inside the bacterial cellwall.Relea

sed into the environment,they can survive extreme heat,lack of water andexposur

e to many toxic chemicals.¡¼¡½

endospores:Ñ¿°û

An endospore is able to remain dormant for a long time,even hundreds ofyears,bu

t it can convert back into vegetative cell in a matter of minutes whencondition

s become more favorable.A bacterium produces a single endospore,whichmerely hel

ps that organism to survive and is not a means of reproduction.Discoveryof bact

erial endospores was of immense important to microbiology.Knowledge ofsuch rema

rkably heat resistant forms was essential for development of adequatemethods o

f sterilization.¡¼¡½

dormant:ÐÝÃßµÄ

vegetative:·±Ö³µÄ

sterilization£ºÃð¾ú

Chapter 3. Growth and Culturing of Bacteria

After completing this chapter you should be able to

¡ñDefine bacterial growth,including binary fission.

¡ñExplain howmicrobes are classified on the basis ofoxygen requiremen

ts.

¡ñDescribe how pure cultures can be isolated.

When we talk about microbial growth,we are really referring to thenumber of cel

ls,not the size of the cells.Microbes that are ¡°growing¡± are increasing in num

ber,accumulating into clumps of hundreds of thousands or populations ofbillions

.For the most part,we are not concerned with the size of an individualcell,beca

use it does not vary much during the cell¡äs lifetime.

By understanding the conditions necessary for microbial growth,we can predictho

w quickly microorganisms will grow in various situations,and determinehow to

c

ontrol their growth.Microbial populations  can become very large in a very sh

ort time,and unchecked microbial growth can cause serious disease andfood spoil

age.

Requirements For Growth

Microorganisms are found in nearly every environment on earth,includingevironme

nts in which no other organisms can survive.The requirements formicrobial growt

h can be divided into two categories£ºphysical and chemical.Physicalaspects inc

lu

de temperature,pH and osmotic pressure.Chemical requirements includewater,sourc

es of carbon and nitrogen,minerals,oxygen and organic growth factors.¡¼¡½

1.Temperature

Microorganisms grow well at rather ordinary temperatures,not muchdifferent from

 those favored by higheranimals.However,certain bacteria are capable of growing

 in extreme cold or extremeheat, at temperatures that would certainly hinder th

e survival of most higher organisms.Microorganisms are divided intothree groups

 on the basis of theirpreferred range of temperature.These are the psychrophile

s (coldª²loving microbes), thermophiles (heatª²loving microbes) andmesophiles (

moderate temperatureª²loving microbes).The optimum temperature for manypathogen

i

c bacteria is about human body (37¡æ),and incubators for clinicalcultures are u

sually set at about this temperature.¡¼¡½

incubator:ÅàÑøÏä

Temperature is important,not only in providing conditions for microbialgrowth,b

ut also in preventing such growth.The refrigeration of food,usually at 4¡æ,redu

ces the growth of psychrophiles and prevents the growth of most otherbacteria.H

owever,food and other materials such as blood can support growth of somebacter

ia even when refrigerated.For this reason,materials that can withstandfreezing

are stored at temperatures of ª²20¡æ if they are to be kept for longperiods of

time.

High temperatures also can be used to prevent bacterial growthlaboratory equipm

ent and media are often sterilized with heat,and food is frequentlypreserved by

 heating and storing inclosed containers.Bacteria are more apt to survive extre

mes of cold than extremes of heat.

2.pH

Most bacteria grow best in a narrow range of pH near neutrality betweenpH6.5 an

d

7.5. Very few bacteria grow at an acid pH below about 4.0.This is thereason why

 a number of foods,such assauerkaraut and cheeses,are preserved by the acids of

 bacterial fermentation.¡¼¡½

sauerkaraut:ÅݲË

When bacteria are cultured in the laboratory,they produce acids that caninterfe

re with desired bacteria growth.To neutralize the acids,chemicals calledbuffers

,are included in the growth medium.The peptones and some amino acids insome med

ia act as buffers,and many media also contain phosphate salts.Phosphatesalts ha

ve the advantage of exhibiting their buffering effect in the pH growthrage of m

ost bacteria.They are also nontoxic;in fact,they provide an essentialnutrients

element.¡¼¡½

buffer:»º³åÒº

peptone:µ°°×ëË

3.Nutrients

Besides water,one of the most important requirements for microbialgrowth is a s

ource of carbon,needed for all the organic compounds that make up aliving cell.

Carbon is the structural backbone of living matter.

In addition to carbon,other important elements are needed by microbesfor the sy

nthesis of cellular material.For example,protein synthesis requiresconsiderable

amounts of nitrogen,as well as some sulfur.The synthesis of DNA and RNAalso req

uire nitrogen and some phosphorus,as does the synthesis of ATP.themolecule so i

mportant for storage transfer of chemical energy within the cell.

4.Oxygen

We are accustomed to think of oxygen as a necessity of life.Organismsthat requi

re oxygen to live are called obligate aerobes.Facultative anaerobes canuse oxyg

en when it is present,but are also to continue growth by usingfermentation or a

naerobia respiration when oxygen is not available.Obligate anaerobes arebacteri

a that are unable to use molecular oxygen for energyª²yieldingreactions.In fact

,most are harmed by it.¡¼¡½

obligate aerobe:רÐÔÐèÑõ¾ú

Facultative anaerobe:¼æÐÔÑáÑõ¾ú

Obligate anaerobe£º×¨ÐÔÑáÑõ¾ú

Growth Of Bacteria

1.Bacteria Division

Bacteria normally reproduce by binary fission,a process in which a celldivides

to produce two equalsized progeny cells.Repeating the process results inthe mul

tiplication of the bacteria population.Bacterial growth proceeds as ageometric

progression,because most bacteria reproduce by binaryfission.Therefore,during a

ctive bacterial growth,the size of the microbial population iscontinously doubl

ing.¡¼¡½

progeny:×Ó´ú

2.Phases Of Growth

A population of organisms introduced into a fresh nutrientª²rich mediumdisplays

 four major phases ofgrowth:(1)the lag phase,(2)the log phase,(3)the stationary

 phase,and (4) the decline ordeath phase.

In the lag phase,the organisms do not increase in number,but they aremetabolica

lly activeª²growing in size,synthesizing enzymes,and incorporatingvarious molec

ules from the medium.During this phase the organisms increase in sizeand they c

apture large quantities of energy in ATP.

Once organisms have adapted to a medium,the cells begin to divide andenter an e

xponential or log growth phase.Because the doubling time is constant,alogarithm

ic plot of growth during the log phase is straight line.During thisperiod,cellu

lar reproduction is most active;on the other hand,microorganisms aremore sensit

ive to adverse conditions than they usually are.Many antimicrobial drugsexert t

heir effect by interfering with some important step in the growthprocess,and ar

e therefore most harmful to cells.

If log growth continues unchecked,some startling numbers of cells canarise.For

example,a single bacterium dividing every 20 minutes for two days cantheoretica

lly produce an ewww.med126.comnormously large population of cells (2114,1 44ª²digit numb

er!).B

ut this does not happen.Eventually,growth slows down,and sooner or laternew cel

ls are produced at the same rate as old cells die,the number of livecells stay

s constant.The culture is then in the stationary phase.

As conditions in the medium become less and less supportive of celldivision,man

y cells lose their ability to divide,and thus the cell die.In this deathphase t

he number of live cells decreases at an exponential rate.

Growth phases are displayed in different ways in colonies growth onsolid medium

.Typically,a cell divides exponentially,forming a small colony atdescendants of

the original cell.The colony grows rapidly at its edges,while cellsnearer its c

enter grow more slowly or begin to die,because they have smallerquantites of nu

trients and are exposed to more waste products.All phases of the growthcurve oc

cur simultaneously in a colony.

Culturing Bacteria

Culturing of bacteria in the laboratory presents two problems.First,apure¡¡cult

ur

e of a single species is needed to study an organism¡äscharacteristics.Second,a

 medium must be found thatwill support the growth of the desired organism.Let u

s look at some of the ways these problems are solved.

1.Isolation Pure Cultures

Most infections materials contain several kinds of bacteria.If we wishto study

a particular microorganism,it is necessary to use procedures forisolating and g

rowing individual colonies of microorganisms.The term clone is used inmicrobiol

ogy as synonymous with pure culture.A clone is a collection of cellswhich are a

ll derived from a single cell.¡¼¡½

clone.colony£º¿Ë¡

The isolation method most commonly used is the streak plate method,whichuses ag

ar

 plates-plates of mediumsoidified with agar and sterilized.Bacteria are picked

up on a sterile wire loop,and the wire is moved lightly along the agar surface,d

e

positing streaks of bacteria on the surface.Fewer and fewer bacteria aredeposit

e

d as the streaking continues,and individual organisms are deposited inthe regio

n streaked last.¡¼¡½

streak plate:ƽ°å»®Ïß

sterilize:Ãð¾ú

After the plate is incubated at a suitable growth temperature for theorganism,s

mall colonies derived from a single organism appear.The wire loop isused to pic

k up a portion of a colony and transfer it to any appropriate sterilemedium for

 further study.¡¼¡½

incubate:·õÓý£¬ÅàÑø

2.Culture Media

A nutrient material prepared for the growth of microorganisms in alaboratory is

 called a culture medium.Somebacteria can grow well on just about any culture m

edium,other require special media,and still others cannot grow on anynonliving

medium yet developed.The microbes that grow and multiply in or on aculture medi

um are referred to as a culture.

Supposed we want to grow a culture of a particular microorganism,perhapsthe mic

robes from a particular clinical specimen.What criteria must the culturemedium

meet? First of all,it must contain the right nutrients for theparticular microo

rganism we want to grow.it should also contain sufficientmoisture,oxygen and a

p

roperly adjusted pH. So that the culture will contain only themicroorganisms we

 add to the medium (and theiroffspring),the medium must initially contain no li

ving microorganisms.Finally,the growing culture should be incubated atthe prope

r temperature.¡¼¡½

specimen:±ê±¾

When it is desirable to grow bacteria on a solid medium,a solidifyingagent such

 as agar is added to themedium,Agar is a complex polysaccharide derived from a

marine alga.Agar has some very important properites that make itvaluable to mic

robiology.Few bacteria can degrade agar,so it remains a solid.Alsoimportant i

s the fact that it melts at about the boiling point of water butremaining in a

liquid state until the temperature,it does not injure bacteria when itis poured

 over a bacterial inoculum ina Petri plate.Bacterial suspensions can also be mi

xed with the melted agar so that a uniform suspension of bacteria ismade;this c

an be used for tests of susceptibility and other purpose.¡¼¡½

alga:º£Ôå

degrade:½µ½â

Petri plate:ÅàÑøÃó

suspension:ÐüÒº

susceptibility:Ãô¸ÐÐÔ

Chapter 4. Control of Microbial Growth

After completing this chapter you should be able to

¡ñDefine sterilization and disinfection.

¡ñDescribe the physical methods of microbial control.

The scientific control of microbial growth began only about one hundredyears ag

o.Prior to that time,it was not uncommon for massive epidemics to killthousands

 of people.In some hospital25% of delivering mothers died of infections carried

 by hands and instruments ofattending nurses and physicians.

Lister read about Pasteur¡äs work with microbes and assumed that thenumber of i

nfected surgical wounds could be decreased through use of proceduresthat preven

ted the access of microbes to the wound.This system,known as asepticsurgery,inc

luded the heat sterilization of surgical instruments and,followingsurgery,the a

pplication of phenol to wounds.The practice so reduced the incidence ofinfectio

us and deaths that other surgeons quickly adopted it ¡¼¡½

aseptic:·À¸¯µÄ

phenol£º±½·Ó

We have come a long way in controlling microbial growth since the timeof Lister

.Today¡äs procedures are far more sophisticated and effective,are usednot only

to control disease organisms,but also to curb microbial growth thatresults in f

ood spoilage.This chapter will discuss how microbial growth can be controlledby

 physical methods andchemical agents.

Sterilization and Disinfection

Sterilization is the killing of all microorganisms in a material or onan object

.When properly carried out, sterilization procedures ensure that evenhighly res

i

stant bacterial endospores and fungal spores are killed.In contrast tosteriliza

tion,disinfection means reducing the number of pathogenic organisms onobjects o

r in materials so that they pose no threat of disease.¡¼¡½

fungal spore:Õæ¾úæß×Ó

disinfection:Ïû¶¾

Physical Methods of Microbial Control

1.Heat

Probably the most common method by which microbes are killed isheat.Heat is the

 most wildly applicable andeffective agent for sterilization.It is also the mos

t economical and easily controlled.Heat appears to kill microbes bydestroying t

heir enzymes by denaturation.The heat used in sterilization can beapplied in th

e form of moist heat or dry heat.¡¼¡½

denaturation:±äÐÔ

One type of moist heat sterilization is boiling.At sea level,boiling(100¡æ) kil

ls vegetative forms of bacterial pathogens,many viruses,and fungi withabout 10

minutes.Freeª²flowing steam is the equivalent in temperature to boilingwater.En

dospores and some viruses,however,are not destroyed quickly.The hepatitisvirus

can survive up to 30 minutes of boiling,and some bacterial endosporeshave resis

ted boiling temperatures for more than 20 hours.Boiling is therefore notalways

a reliable sterilization procedure.However,a few minutes of boilingtemperatures

 will kill most pathogens andgenerally make food or water safe to eat or drink.

 ¡¼¡½

vegetative form:·±Ö³Ìå

To reliably sterilize with moist heat,temperatures above that of boilingwater a

re needed.This high temperature is most commonly achieved by steam underpressur

e in an autoclave.The preferred method of sterilization,autoclaving isused unle

ss the material to be sterilized would be damaged by heat ormoisture.The higher

the pressure in the autoclave,the higher the temperature.Forexample,when freeª²

flowing steam,at a temperature of 100¡æ,is placed under a pressure of 1atmosphe

re above sea level pressure,that is ,above 15 pounds per square inch,thetempera

ture is increased to 121¡æ.Steam at 121¡æ will kill all organisms andtheir endo

spores in about 15 minutes.or perhaps a bit longer,depending on type andvolume

of material being sterilized.¡¼¡½

autoclave:¸ßѹÃð¾ú¹ø

Autoclaving is used to sterilize culturemedia,instruments,dressings,solutions,s

y

ringes and numerous other items that can withstand high temperature andpressure

.¡¼¡½

solutions:ÈÜÒº

syringe£º×¢ÉäÆ÷

One of the simplest methods of dry heat sterilization is directflaming.You will

 use this procedure manytimes in the laboratory when you sterilize inoculating

loops and needles.To sterilize the inoculating loop or needle,all youhave to do

is to heat the wire to a red glow.This method is 100% effective.Asimilar princi

ple is used in incineration.This is an effective way to sterilize anddispose of

 contaminated paper cups,bagsand dressings.¡¼¡½

flaming:ÓûðÑæÉÕ

inoculating loop:½ÓÖÖ»·

incineration:·ÙÉÕ

In the early days of microbiolgy,Pasteur found a practical method ofpreventing

 the ¡°sickness¡± or spoilage of beer andwine.Pasteur used mild heating,which w

as sufficient to kill the organisms that caused the particular spoilageproblem

without seriously damaging the taste of the product.The same principlewas later

 applied to milk.In classicpasteurization treatment of milk,the milk was expose

d to a  temperature of about63¡æ for 30 minutes.Most pasteurization done today

uses higher temperatures,at least 72¡æ,for about 15 seconds.Thistreatment,known

 as highª²temperature,shortª²timepasteurization,is applied as the milk flows co

ntinuously past a heat exchanger.

2.UV Light

UV light damages the DNA of exposed cells.It causes bonds to formbetween adjace

nt thymines in DNA chains.These thymine dimmers inhibit correctreplication of t

he DNA during reproduction of the cell.The UV wave lengths mosteffective or kil

ling microbes are around 260nm; these wave lengths are specificallyabsorbed by

cellular DNA.UV radiation is also used to control microbes in the air.UVor ¡°ge

rmicidal¡± lamp iscommonly found in hospital rooms,nurseries and operating room

s.¡¼¡½

thymines:ÐØÏÙà×à¤

replication:¸´ÖÆ

germicidal:ɱ¾úµÄ

A major drawback of UV light as a sterilize is that it has lowpenetrability,so

organisms to be killed must be directly exposed to the rays.Organismsprotected

by solids and coverings such as paper,glass and textiles are notaffected. Anoth

er potential problem is that UV light can damage the eyes,and prolongedexposure

 to UV light can cause burnsand skin cancer.¡¼¡½

penetrability:´©Í¸Á¦

Chemical Methods of Microbial Control

Chemical agents are used to control microbes on living tissue andinanimate obje

cts.Unfortunately,few chemical agents achieve sterility;most of themmerely red

uce microbial populations to safe levels or remove vegetative forms ofpathogens

 from objects.A commonproblem in disinfection is the selection of an agent that

 will kill all organisms inthe shortest period of time without damaging the con

taminated material.Just as there is no single physical method ofmicrobial contr

o

l that can be used in every situation,there is no one disinfectant thatwill be

appropriate for all circumstances.¡¼¡½

inanimate:ÎÞÉúÃüµÄ

sterility:ÎÞ¾ú

disinfectant:Ïû¶¾¼Á

The more of the following qualities a disinfectant has,the moreeffective it is:

1.Acts rapidly.

2.Attacks all or a wide range of microbes.

3.Is able to penetrate thoroughly the material that is contaminated.

4.Readily mixes with water to form a stable solution or emulsion.

5.Is not hampered by organic matter on the substance to be disinfected

.

6.Is not likely to decompose and thereby lose its activity after expos

ure to light,heat or unfavorable weather.

7.Does not stain,corrode or destroy the object being disinfected.

8.Is harmless to animals if it is to be used as antiseptic and does no

t destroy body tissues or act as a toxin if inhaled or swallowed.¡¼¡½

decompose:·Ö½â

corrode:¸¯Ê´

inhale:ÎüÈë

The ideal disinfectant should also have a pleasant odor,and beeconomical to use

 and safe to transport.

We should also remember that the concentration of a disinfectant willaffect its

action.A disinfectant should always be diluted exactly as suggested bythe manuf

acturer.Solution that are too weak may be ineffective,or bacteriostaticinstead

of bactericidal.On the other hand,solutions that are too strong can bedamagerou

s

 to humans who come incontact with them.For example,the recommended optimum con

centration of ethanol is 70%.¡¼¡½

bacteriostatic:ÒÖ¾úµÄ

bactericidal£ºÉ±¾úµÄ

Chapter 5. Mechanisms of Pathogencity

After completing this chapter you should be able to

¡ñDefine pathogencity and virulence.

¡ñExplain how adherence,capsule,cell wall components andenzymes contr

ibute to pathogenicity.

¡ñContrast the nature and effects of exotoxin andendotoxin.

Now that you have a basic understanding of the structural and functionalaspect

s of microorganisms,we can consider the ways in which the human body andmicroor

ganisms are related in terms of health and disease.

We all have defense mechanisms that are always operating to keep ushealthy.For

instance,unbroken skin and mucos membranes are effective barriersagainst microb

ial invasion and infection.Tears,saliva and gastric juice also playimportant de

fensive roles.The inflammatory response,phagocytosis and fever areadditional at

tempts to keep us healthy.And under certain conditions,we can produceproteins c

alled antibodies,which combine with particular microorganisms andcontribute t

o their destruction.However microorganisms have properties that makethem pathog

enic,that is capable of causing disease.¡¼¡½

mucos membrane:ճĤ

saliva:ÍÙÒº

gastric juice:θҺ

A rather delicate balance exists between our defenses and the diseaseª²producing

mechanisms of microorganisms.When our defenses resist the diseaseª²producingcap

ab

ilities of the microorganism,we maintain our health.But when the abilityof the

microorganism to cause disease overcomes our defenses.diseaseresults.After the

disease has become established,an infected person can recover completely£¬suffe

r

permanent damage or die,depending on many factors.

Any parasite capable of causing disease in its host is called apathogen.Pathoge

ns vary in their abilities to disturb health,that is,they displaydifferent degr

ee of pathogenicity.Pathogenicity is the capacity to produce disease ina host.O

n this basis,bacteria can be organized into three major groups£ºfrankpathogen,

opportunistic pathogen and nonpathogen.¡¼¡½

parasite:¼ÄÉú³æ(¾ú)

pathngan:²¡Ô­Ìå

pathogenicity:Ö²¡ÐÔ

opportunistic pathogen:»ú»áÖ²¡¾ú

When isolated from a patient,frank pathogen are considered to beprobable agents

 of disease (e.g. when thecause of diarrheal disease is identified by the labor

atory isolation of Salmonellaspp from feces). Opportunistic pathogen arethose i

solated from patients whose host defense mechanisms have beencompromised.They m

ay be the agents of disease (e.g.in patients who have been predisposedto urinar

y trace infections with E.coli by catheterization). Finally,somebacteria,such a

s Lactobacillus acidophilus,are conisdered to be nonpathogen,becausethey rarely

or never cause human disease.Their categorization as nonpathogens maychange,how

ever,because of the adaptability of bacteria and the detrimental effectof moder

n radiaiton therapy,chemotherapy,and immunotherapy on resistancemechanisms.In f

a

ct,some bacteria previously considered to be nonpathogens are now knownto cause

 disease.¡¼¡½

diarrheal:¸¹ÐºµÄ

Salmonella:ɳÃÅÊϾúÊô

urinary trace:Äò·

catheterize:µ¼¹Ü²åÈë

Lactobacillus acidophilus:ÊÈËáÈé¸Ë¾ú

Virulence is the degree of pathogenicity.It is affected by numerousvariables su

c

h as the number of infecting bacteria,route of entry into thebody,virulence fac

tors of the bacterium and specific and nonspecific host defensemechanisms.

In order for a pathogen to cause disease,it must gain access to thehost,adhere

to host tissues,resist or evade host defenses,multiply and damage thehost tissu

e. ¡¼¡½

tissue£º×éÖ¯

How Pathogens Adhere To Host Tissues

To cause infection,many bacteria must first adhere to cells,usuallyepithelial c

ells.If they did not adhere,they would be swept away by mucus and otherfluids t

hat bathe the tissue surface.Many bacteria have evolved attachmentmechanisms,su

ch as pili and hairª²like structures (lipoteichoic acid and Mprotein),that reco

gnize and attach the bacteria to cells.For many diseaseª²causingbacteria,invasi

on of the host¡äs epithelium is central to the infectious process.Somebacteria

(e.g.Sallmonella species) invade tissues through the junctions betweenepithelia

l cells.Other bacteria (e.g.N.gonorrhoeae) invade the host¡äs epithelialcells a

nd many subsequently enter the tissues.Some bacteria (e.g.Shigella species) mul

tiply within host cell,whereas other bacteria (e.g.Yersinia enterocolitica)do n

ot.¡¼¡½

adhere:Õ³¸½

epithelial:ÉÏƤµÄ

attach:Õ³¸½

invasion:ÈëÇÖ

N.gonorrhoeae:ÁÜÇò¾ú

Shigella:Ö¾ºØÊϾúÊô

How Pathogens Resist Host Defenses

1.Surface components

Capsules resists the host¡äs defenses by impairing phagocytosis,a processby whi

ch certain cells of the body engulf and destroy microbes.It appears thatthe che

mical nature of the capsule prevents the phagocytic cell from adheringto the ba

cterium.Streptococcus contains a heatª²andª²acidª²resistant protein calledM pro

tein,which helps the bacterium resist phagocytosis by white bloodcells.Sallmonella typhi carries a surface antigen,the Vi antigen,thoug

ht to enhance virulence.

Some bacteria have the ability to survive and multiply inside phagocyticcells.A

 classic example isMycobacterium tuberculosis,whose sur

viva

l seems to depend on the structure and composition of its cell surface.¡¼¡½

engulf:ÍÌʳ

Sallmonella typhi:É˺®¸Ë¾ú

Mycobacterium tuberculosis:½áºË¸Ë¾ú

2.Enzymes

The virulence of some bacteria is thought to be aided by the productionof extra

cellular enzymes and related substances.These chemicals promotespreading of bac

teria in tissues by dissolving hyaluronic acid between cells(Hyaluronidase), ca

use localization of bacteria in tissue by forming blood clot whichprevents host

 cells from attacking(Coagulases),dissolve blood clots (Streptokinase) and degr

ade DNA (DNAases).¡¼¡½

hyaluronidase:͸Ã÷ÖÊËáø

coagulases:Ѫ½¬Äý¹Ìø

streptokinase:Á´¼¤Ã¸

3.Antigenic variation

Microbes avoid host immune responses by varying surface antigens.Nei

sseria genorrhoeae express several different P¢ò proteins at any giv

en time,thereby preventi

ng the host from making ¡°speciesª²specific¡±P¢ò protein antibodies.

4.IgA proteases

Neisseria genorrhoeae and H.influenza produc

e enzymes which are capable of cleav

ing secretary IgA antibodies,which presumably enhances the ability of anbacteri

um to survive.

How Pathogens Damage To Host Cells

The ways in which pathogens bring about damage to the host arediverse.Only rare

ly are symptoms of a disease due to the presence of large numbers ofmicroorgani

sms per se.Pathogenic bacteria cause damage to host cells in three basicways:

¡¼¡½

per se£º±¾Éí

1.by causing direct damage in the immediate vicinity of the invasion;

2.by producing toxins,poisonous substances transported by blood and ly

mph that damage sites far removed from the original site of invasion;

3.by inducing hypersensitivity reaction.¡¼¡½

toxin:¶¾ËØ

hypersensitivity reaction:±ä̬·´Ó¦

Most damage by bacteria is done by toxin.Toxins may be almost entirelyresponsib

le for the pathogenic properties of the bacteria.Toxins are of twotypes:EXOTOXI

N and ENDOTOXINS.¡¼¡½

exotoxin:ÍⶾËØ

endotoxin:ÄÚ¶¾ËØ

1.Exotoxin

Some bacteria produce exotoxins as part of their growth and metabolismand relea

se the exotoxins into the surrounding medium.Most bacteria that produceexotoxin

s

 are gramª²positive.Exotoxinsare proteins.Most exotoxins consist of two moietie

s:one aids entrance of the exotoxin into the host cells,and the otherprovides t

h

e toxic activity.Because they are soluble in body fluids,exotoxinseasily diff

use into the blood and are rapidly transported throughout the body.¡¼¡½

moiety:µ¥Ìå

Exotoxins work by destroying particular parts of the host¡äs cells or byinhibit

ing certain metabolic functions.Exotoxin can be grouped into severalcategorie

s based on their biologic effect¡¡on host cells.Neurotoxins are bestexemplified

by the botulinum toxin formed by C.botulinum.This potent neurotoxin actson moto

r neurons by preventing the releases of acetylcholine at the myoneuraljuncting,

thereby preventing muscle excitation and producing flaccidparalysis.Diphtheria

is one of cytotoxins which inhibits protein synthesis in many celltypes.Enterot

oxin stimulate hypersecretion of water and electrolytes from theintestinal epit

helium and thus produce watery diarrhera.¡¼¡½

Neurotoxin:Éñ¾­¶¾ËØ

C.botulinum:Èⶾ¸Ë¾ú

acetylcholine:ÒÒõ£µ¨¼î

flaccid paralysis:¾·ÂÎ

Diphtheria:°×ºí¶¾ËØ

cytotoxin:ϸ°û¶¾ËØ

enterotoxin:³¦¶¾ËØ

electrolyte£ºµç½âÖÊ

Exotoxins are among the most lethal substances known,and 1 mg ofbotulinum toxin

 is enough to kill 1 millionguineaª²pigs.For example,tetanus caused by the toxi

n of Glostridium tetani killed as many as 50,000 soldier

s of the Axis powers in

world ¢ò;the Allied forces,however,immunized soldiers against tetanus,andvery f

ew of their soldiers died of this disease.Fortunately,only a fewbacteria produc

e exotoxins.¡¼¡½

tetanus:ÆÆÉË·ç

C.tetani:ÆÆÉË·ç¸Ë¾ú

The body produces antibodies called antitoxins that provide immunity toexotoxin

s.The antitoxins can react with the exotoxin and neutralize it,whereasantibody

to endotoxin reacts with the LPS complex but has little effect on thebiologic a

ctivity of the complex,except under special conditions.¡¼¡½

antitoxin:¿¹¶¾ËØ

When exotoxins are inactivated by heat or formaldehyde,they no longercause the

disease but are still able to stimulate the body to produceantitoxins.Such alte

red exotoxins are called toxoids.When toxoids are injected into thebody,they st

imulate antitoxin production so that immunity is produced to diseases.¡¼¡½

toxoid:À඾ËØ

2.Endotoxin

Endotoxins differ from exotoxins in several ways.Endotoxins are part ofthe oute

r

portion of the cell wall of most gramª²negative bacteria.The gramª²negativebact

eria have an out membrane surrounding the peptidoglycan layer of thecell wall.T

his out membrane consists of lipoproteins,phospholipids andlipopolysaccharides

(LPS). The lipid portion of LPS,called lipid A,conferstoxicity.Thus,endotoxins

are LPS,whereas exotoxins are proteins.Endotoxins exert their effectswhen the g

ramª²negative bacteria die and their cell walls undergo lysis,thusliberating th

e endotoxin.

All endotoxins produce the same signs and symptoms,regardless of thespecies of

microorganism,although not to the same degree.Fever is an almostuniversal sympt

om of endotoxemia,because endotoxin stimulates host cells to releaseproteins ca

lled pyrogens,which affect the temperatureª²center of the brain.Largedose of en

dotoxin can cause death.Thus,endotoxin must be removed from all medicalsupplies

 destined for injection onuse during surgical procedures.¡¼¡½

pyrogens:ÈÈÔ­ÖÊ

(Áú±±¹ú)

Ó¢ Óï ר Òµ ´Ê »ã ±í

abortive infection¶Ù´ì¸ÐȾ

Viral infection in which viruses enter in a cell but are unable toexpress all t

heir genes to make infectious progeny.

adherenceÕ³¸½

The attachment of a microorganism to a host cell surface.

aerobesÐèÑõ¾ú

Organisms that use oxygen,including some that must have oxygen.

agar plateÇí֬ƽ°å

A plate of medium solidified with agar,a polysaccharide extracted fromcertain m

arine algae.

anaerobesÑáÑõ¾ú

Organisms that do not use oxygen,including some that are killed byexposure.

antibiotic ¿¹ÉúËØ

A chemical substance produced by microorganisms that can inhibit thegrowth of o

r destroy other microorganisms.

antimicrobial agent¿¹Î¢ÉúÎï¼Á

A chemotherapeutic agent used to treat diseases caused by microbes.

antisepsis·À¸¯

A chemical agent that can be safely used externally on tissues todestroy microo

rganisms or to inhibit their growth.

antitoxin¿¹¶¾ËØ

An antibody against a specific toxin.

antiviral protein¿¹²¡¶¾µ°°×

Protein that interferes with the replication of viruses.

aseptic techniquesÎÞ¾ú²Ù×÷

Techniques used to minimize chances that cultures will be contaminated byorgani

sms from the environment.

bacillus (¸´Êý:bacilli)¸Ë¾ú

A rodlike bacterium.

bacteremia¾úѪ֢

An infection in which bacteria are transported in the blood but do notmultiply

in transit.

bacterium(¸´Êý£ºbacteria)ϸ¾ú

bacteriocinϸ¾úËØ

Proteins released by some bacteria that inhibit the growth of otherstrains of t

he same or closely related species.

bacteriophageÊɾúÌå

A virus that infects bacteria.

binary fission¶þ·ÖÁÑ

Process in which a bacterial cell duplicates its components and dividesinto two

 cells.

blood agarѪƽ°å

Type of medium containing sheep blood,used to identify organisms thatcause hemo

lysis,or breakdown of red blood cells.

capsidÒ¿ǡ¢ºË¿Ç

The protein coating of a virus,which protects the nucleic acid core fromthe env

ironment and determines the shape of the virus.

capsule¼ÔĤ

A protective structure outside the cell wall,secreted by the organism.

cell wallϸ°û±Ú

Outer layer of most bacterial,algal,fungal,and plant cells thatmaintains the sh

ape of the cell.

chemotherapy»¯Ñ§ÁÆ·¨

The use of chemical substances to treat various aspects of disease.

chlamydiaÒÂÔ­Ìå

Tiny,nonmotile,spherical bacterial; all are obligate intracellularparasites wit

h a complex life cycle.

chronic infectionÂýÐÔ¸ÐȾ

coagulaseѪ½¬Äý¹Ìø

Bacterially produced enzyme that accelerates the coagulation of blood.

coccus(¸´Êý£ºcocci)Çò¾ú

A spherical bacterium.

colonization¶¨Ö²

Growth of microorganisms on epithelial surfaces such as skin or mucousmembranes

.

colony¾úÂä

A group of descendants of an original cell.

conjugation½ÓºÏ

Transfer of genetic information from one bacterial cell to another bymeans of p

ili.

contagious disease´«È¾²¡

Infectious disease that can be spread from one host to another.

cultureÅàÑø£¬ÅàÑøÎï

cytotoxinϸ°û¶¾ËØ

An kind of exotoxin produced by bacteria.

decline phaseË¥ÍöÆÚ

The fourth of four major phases of the bacterial growth curve in whichcells los

e their ability to divide (due to less supportive conditions in themedium) and

thus die.

disinfectantÏû¶¾¼Á

A chemical agent used on inanimate objects to destroy microorganisms.

disinfectionÏû¶¾

Reducing the number of pathogenic organisms on objects or in materialsso that t

hey pose no threat of disease.

endospore Ñ¿°û

A resistant,dormant structure,formed inside some bacteria such asBacillus and C

lostridium,that can survive adverse conditions.

endogenous infectionÄÚÔ´ÐÔ¸ÐȾ

An infection caused by microorganisms that exist in the body.

endotoxinÄÚ¶¾ËØ

A toxin incorporated in gramª²negative bacterial cell walls and releasedwhen th

e bacterium dies.

enterotoxin³¦¶¾ËØ

An exotoxin that acts on tissues of the gut.

envelope°üĤ

A bilayer membrane found outside the capsid of some viruses, acquired asthe vir

us buds through the host cell¡äs membrane.

exogenous infectionÍâÔ´ÐÔ¸ÐȾ

An infection caused by microorganisms that enter the body from the  environment.

exotoxinÍⶾËØ

A soluble toxin secreted by microbes into their surroundings,includinghost tiss

ues.

exponential growth¶ÔÊýÉú³¤ÆÚ

Growth of a bacterial culture characterized by doubling of thepopulation in a f

ixed interval of time.

facultative anaerobee¼æÐÔÑáÑõ¾ú

Bacteria that carry on aerobic metabolism when oxygen is present butshift to an

aerobic metabolism when oxygen is absent.

fermentation·¢½Í

Anaerobic metabolism of the pyruvate produced in glycolysis.

flagellum (¸´Êý£ºflagella)±Þë

Long,thin,helical appendages of certain cells that provide means oflocomotion.

flora disequilibrium¾úȺʧµ÷

fungus(¸´Êý£ºfungi)Õæ¾ú

Nonphotosynthetic,eukaryotic organisms that absorb mutrients from theirenvironm

ent.

germ²¡Ô­Î¢ÉúÎï(¾ú)

Pathogenic microorganisms.

Gram stain¸ïÀ¼ÊÏȾɫ·¨

A differential stain.Gramª²positive bacteria stain dark purple,gramª²negativeon

es stain pink/red.

growth curveÉú³¤ÇúÏß

The different growth periods of a baterial or phage population.

hepatitis¸ÎÑ×

An inflammation of the liver,usually caused by viruses but sometimes byan amoe

ba or various toxic chemicals.

helper virus.¸¨Öú²¡¶¾

horizontal transmissionˮƽ´«²¥

host¼ÄÖ÷

Any organism that harbors another organism.

hypha(¸´Êý£ºhyphae)¾úË¿

Long,threadlike structures of cells in fungi or actinomycetes.

infection¸ÐȾ

The multiplication of a parasite,usually microscopic,within the body.

inoculation½ÓÖÖ

interferon¸ÉÈÅËØ

A small protein released from virusª²infected cells which binds adjacentcells,c

ausing them to produce a protein that interferes with viral replication.

invasivenessÇÖÏ®Á¦

The ability of a microorganism to take up residence in a host.

L formsLÐÍϸ¾ú

Irregularly shaped naturally occurring bacteria with defective cellwalls.

lag phase³Ù»ºÆÚ

First of four major phases of the bacterial growth curve,in whichorganisms grow

 in size but do not increasein number.

latent infectionDZ·ü¸ÐȾ

lipidÖ¬ÖÊA¡¢ÀàÖ¬A

Toxic substance found in the cell wall of gramª²negative bacteria.

lipopolysaccharide(LPS)Ö¬¶àÌÇ

Part of the outer layer of the cell wall in gramª²negative bacteria.

logarithmic phase¶ÔÊýÆÚ

Second of four major phases of the bacterial growth curve,in which cellsdivide

at an exponential or logarithmic rate.

lysozymeÈܾúø

An enzyme that acts on polysaccharides to weaken the bacterial cellwalls.

medium(¸´Êý£ºmedia)ÅàÑø»ù

microaerophilic bacteria΢ÐèÑõ¾ú

Bacteria that grow best in the presence of a small amount of freeoxygen.

microbe΢ÉúÎï

microorganism΢ÉúÎï

microscopeÏÔ΢¾µ

microbiology΢ÉúÎïѧ

The study of microorganisms.

mycoplasmasÖ§Ô­Ìå

Very small bacteria with cell membranes,RNA and DNA,but no cell walls.

neurotoxinÉñ¾­¶¾ËØ

A toxin that acts on nervous system tissues.

normal flora Õý³£¾úȺ

Microorganisms commonly found in or on another organism.

nosocomial infection Ò½ÔºÄÚ¸ÐȾ

An infection acquired in a hospital or other medical facility.

nucleoidÄâºË

Nuclear region in bacteria.

nucleocapsidºËÒ¿Ç

obligate aerobeרÐÔÐèÑõ¾ú

Bacteria that must have free oxygen to grow.

obligate anaerobeרÐÔÑáÑõ¾ú

Bacteria that are killed by free oxygen.

opportunistic bacterium»ú»áÖ²¡¾ú

Species of resident of transient flora that do not ordinarily causedisease but

can do so under certain conditions.

outer membraneÍâĤ

A bilayer membrane,surrounding the cell wall of gramª²negative bacteria.

pasteurization°Í˹µÂÏû¶¾·¨

Mild heating to destroy pathogens and other organisms that causespoilage.

pathogen²¡Ô­Ìå

Any parasite capable of causing disease in its host.

pathogenic bacterium²¡Ô­¾ú

pathogenicityÖ²¡ÐÔ

The capacity to produce disease.

penicillinÇàùËØ

An antimicrobial agent made by the mold Penicillium.

peptidoglycanëľÛÌÇ

A structural polymer in the bacterial cell wall that forms a supportingnet.

peptoneµ°°×ëË

A product of enzyme digestion of proteins that contains many small peptides;a c

ommon ingredient of a complex medium.

persistent infection³ÖÐø¸ÐȾ

phage See bacteriophageÊɾúÌå

phagocytosisÍÌÊÉ×÷ÓÃ

Digestion of solids into cells by means of the formation of vacuoles.

pigmentÉ«ËØ

pilus(¸´Êý£ºpili)¾úë

Tiny hollow projections used to attach bacteria to surfaces (fimbriae)of for co

njugation( sex pili).

plasmidÖÊÁ£

A small circular piece of DNA in a cell that is not part of itschromosome.

plateƽ°å

prionÀಡ¶¾

An exceedingly small infectious particle alleged to consist of proteinwithout a

ny nucleic acid; their existence has been challenged by someinvestigators.

prophageÇ°ÊɾúÌå

Phage DNA that has entered a bacterium.

protozaÁ¢¿Ë´ÎÌå

Singleª²celled,microscopic,eukaryotic organisms in kingdom Protista.

pure culture´¿ÅàÑø

A culture that contains only a single species of organism.

pyemiaŧ¶¾ÑªÖ¢

pyrogenÈÈÔ­ÖÊ

A substance that acts on the hypothalamus to raise the body¡äs ¡°thermostat¡± to

 a higherª²thanª²normaltemperature.

R factors (plasmids)RÒò×Ó¡¢RÖÊÁ£

Drug resistance plasmids,which carry genes that provide resistance tovarious an

tibiotics.

replication¸´ÖÆ

Process by which an organism of structure (especially a DNA molecule)duplicates

 itself.

reproduce·±Ö³

reservoir of infection´«È¾Ô´

Sites where microorganisms can persist and transmission to other hosts.

resident flora³£¾Ó¾úȺ

Species of microorganisms that are always present on or in an organism.

resistanceÄÍÒ©ÐÔ

The ability of a microorganism to remain unharmed by an antimicrobialagent.

resistance transfer factor ÄÍÒ©ÐÔ´«µÝÒò×Ó

Component of a resistance plasmid that implements transfer by conjugationof the

 plasmid.

rickettsiaÁ¢¿Ë´ÎÌå

Small,nonmotile,gramª²negative organisms;obligate intracellular parasitesof ma

mmalian and arthropod cells.

septicemia°ÜѪ֢

An infection caused by rapid multiplication of pathogens in theblood;formerly c

alled blood poisoning.

slow viral infectionÂý(·¢)²¡¶¾¸ÐȾ

smearͿƬ

A thin layer of liquid specimen spread out on a microscope slide.

spirillum(¸´Êý£ºspirilli)Âݾú

A corkscrewª²shaped bacterium

spike´ÌÍ»

spirocheteÂÝÐýÌå

A flexible,wavyª²shaped bacterium.

sporeæß×Ó

A resistance reproductive structure formed by fungi andactinomycetes;different

from bacterial endospores.

stainȾɫ

A molecule that can bind to a structure and give it color.

staphylococcus aureus½ð»ÆÉ«ÆÏÌÑÇò¾ú

staphylococcus epidermidis±íƤÆÏÌÑÇò¾ú

stationary phaseÎȶ¨ÆÚ

The third of four major phases of the bacterial growth curve in whichnew cells

are produced at the same rate that old cells die,leaving the number oflive cell

s constant.

sterilityÎÞ¾ú

The state in which there are no living organisms in or on a material.

sterilizationÃð¾ú

The killing or removal of all microorganisms in a material or on anobject.

strain¾úÖê

A subgroup of a species with one or more characteristics thatdistinguish it fro

m other subgroups that species.

streak plate methodƽ°å»®Ïß·ÖÀë·¨

Method used to prepare pure cultures in which bacteria are lightlyspread over t

he surface of agar plates,resulting in isolated colonies.

superinfection¶þÖظÐȾ

Invasion of digestive,respiratory,or urinary tracts by resistancereplacement fl

ora when normal flora are disturbed.

temperate phageκÍÊɾúÌå

A phage that ordinarily does not cause a virulent infection but ratheris incorp

orated into a bacterium and replicated with it.

toxemia¶¾ÑªÖ¢

The presence of pathogenª²released toxins in the blood.

toxin¶¾ËØ

A poisonous substance.

toxoidÀ඾ËØ

An exotoxin inactivated by chemical treatment which retains itsantigenicity and

 therefore can be used tovaccinate against the toxin.

transductionתµ¼

The transfer of genetic material from one bacterium to another by abacteriophag

e.

transformationת»¯

A change in an organism¡äs characteristics through the transfer of nakedDNA.

transient flora¹ý·¾úȺ

Microorganisms that may be present in or on an organism under certainconditions

 and for certain lengths oftime.

transposonת×ù×Ó

vaccineÒßÃç

A substance that contains an antigen to which the immune systemresponds.

vegetative form·±Ö³Ìå

Cells that are actively metabolizing nutrients.

vertical transmission´¹Ö±´«²¥

Direct contact transmission of disease in which pathogens are passedfrom parent

 to offspring in an egg orsperm,across the placenta,or while traversing the bir

th canal.

vibrio»¡¾ú

A commaª²shaped bacterium.

viremia²¡¶¾ÑªÖ¢

An infection in which viruses are transported in the blood but do notmultiply i

n transit.

virion²¡¶¾¿ÅÁ£

A complete virus particle,including its envelop if it has one.

virology²¡¶¾Ñ§

The study of viruses.

virulence¶¾Á¦

The degree of intensity of the disease produced by a pathogen.

virulent phage ÁÒÐÔ»ò¶¾ÐÔÊɾúÌå

A phage that enters the lytic cycle when it infects a cell,causingeventual deat

h of the cell.

virus²¡¶¾

Submicroscopic,parasitic,acellular entities composed of a nucleic acidcore insi

de a protein coat.

yeast½Íĸ¾ú

(Áú±±¹ú)