Medical Microbiology

3 GeneralBacteriology

F.H.Kayser

TheMorphologyandFineStructureofBacteria

&Bacterialcellsarebetween0.3and 5 lminsize.Theyhavethreebasic

forms:cocci,straightrods,andcurvedorspiralrods.Thenucleoidconsists

ofaverythin,long,circularDNAmoleculardoublestrandthatisnotsur-

roundedbyamembrane.Amongthenonessentialgeneticstructuresare

theplasmids.The cytoplasmicmembraneharborsnumerousproteins

suchaspermeases,cellwallsynthesisenzymes,sensorproteins,secretion

systemproteins,and,inaerobicbacteria,respiratorychainenzymes.The

membraneissurroundedbythecellwall,themostimportantelementof

whichisthesupportingmureinskeleton.ThecellwallofGram-negativebac-

teriafeaturesaporousoutermembraneintotheoutersurfaceofwhichthe

lipopolysaccharideresponsibleforthepathogenesisofGram-negativeinfec-

tionsisintegrated.ThecellwallofGram-positivebacteriadoesnotpossess

suchanoutermembrane.Itsmureinlayeristhickerandcontainsteichoic

acidsandwall-associatedproteinsthatcontributetothepathogenicprocess

inGram-positiveinfections.Manybacteriahavecapsulesmadeofpolysac-

charidesthatprotectthemfromphagocytosis.Attachmentpiliorfimbriae

facilitateadhesiontohostcells.Motilebacteriapossessflagella.Foreignbody

infectionsarecausedbybacteriathatformabiofilmoninertsurfaces.Some

bacteriaproducespores,dormantformsthatarehighlyresistanttochemical

andphysicalnoxae. &

BacterialForms............................................

Bacteriadifferfromothersingle-cellmicroorganismsinboththeircell

structureandsize,whichvariesfrom0.3– 5 lm.Magnificationsof 500 –

1000 !—closetotheresolutionlimitsoflightmicroscopy—arerequiredto

obtainusefulimagesofbacteria.Anotherproblemisthatthestructuresof

objectsthesizeofbacteriaofferlittlevisualcontrast.Techniqueslikephase

contrastanddarkfieldmicroscopy,bothofwhichallowforlivecellobserva-

tion,areusedtoovercomethisdifficulty.Chemical-stainingtechniquesare

alsoused,butthepreparedspecimensaredead.

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Kayser, Medical Microbiology © 2005 Thieme