Medical Microbiology

182 3 GeneralBacteriology

Bacteriophages.....................................................

&Bacteriophages,orsimplyphages,arevirusesthatinfectbacteria.They

possessaproteinshellsurroundingthephagegenome,whichwithfewex-

ceptionsiscomposedofDNA.Abacteriophageattachestospecificreceptors

onitshostbacteriaandinjectsitsgenomethroughthecellwall.Thisforces

thehostcellstosynthesizemorebacteriophages.Thehostcelllysesattheend

ofthisreproductivephase.So-calledtemperatebacteriophageslysogenize

thehostcells,wherebytheirgenomesareintegratedintothehostcellchro-

mosomesastheso-calledprophage.Thephagegenesareinactiveinthis

stage,althoughtheprophageisduplicatedsynchronouslywithhostcellpro-

liferation.Thetransitionfromprophagestatustothelyticcycleistermed

spontaneousorartificialinduction.Somegenomesoftemperatephages

maycarrygeneswhichhavethecapacitytochangethephenotypeofthe

hostcell.Integrationofsuchaprophageintothechromosomeisknown

aslysogenicconversion. &

Definition..................................................

Bacteriophagesarevirusesthehostcellsofwhicharebacteria.Bacterio-

phagesarethereforeobligatecellparasites.Theypossessonlyonetypeof

nucleicacid,eitherDNAorRNA,havenoenzymaticsystemsforenergysupply

andareunabletosynthesizeproteinsontheirown.

Morphology................................................

Similarlytothevirusesthatinfectanimals,bacteriophagesvarywidelyinap-

pearance.Fig.3.25ashowsaschematicviewofaTseriescoliphage.Research

onthesephageshasbeenparticularlythorough.Fig.3.25bshowsanintact

Tphagenexttoaphagethathasinjecteditsgenome.

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Kayser, Medical Microbiology © 2005 Thieme