Medical Microbiology

186 3 GeneralBacteriology

Lysogeny...................................................

Fig.3. 26 illustratesthelysogenyofahostcell.Followinginjectionofthe

phagegenome,itisintegratedintothechromosomebymeansofregion-spe-

cificrecombinationemployinganintegrase.Thephagegenomethusinte-

gratediscalledaprophage.Theprophageiscapableofchangingtothe

vegetativestate,eitherspontaneouslyorinresponsetoinductionbyphysical

orchemicalnoxae(UVlight,mitomycin).Theprocessbeginswithexcisionof

thephagegenomeoutoftheDNAofthehostcell,continueswithreplication

ofthephageDNAandsynthesisofphagestructureproteins,andfinallyends

withhostcelllysis.Cellscarryingaprophagearecalledlysogenicbecause

theycontainthegeneticinformationforlysis.Lysogenyhasadvantagesfor

bothsides.Itpreventsimmediatehostcelllysis,butalsoensuresthatthe

phagegenomereplicatesconcurrentlywithhostcellreproduction.

Lysogenicconversioniswhenthephagegenomelysogenizingacellbearsa

gene(orseveralgenes)thatcodesforbacterialratherthanviralprocesses.

Geneslocalizedonphagegenomesincludethegenefordiphtheriatoxin,

thegeneforthepyrogenictoxinsofgroupAstreptococciandthecholeratoxin

gene.

TheImportanceoftheBacteriophages

Biologicalresearch Bacteriophagesareoftenusedasmodelsinstudiesof

fundamental biological processes: DNA replication,

geneexpression,generegulation,viralmorphogenesis,

studiesofthedetails,andfunctionofsupramolecular

structures

Geneticengineering Vectorsforgenecloning,adjuvantsinsequencing

Therapyandprevention Anolderconceptnowreceivingincreasedattention.Ad-

ministrationofsuitablephagemixturesintherapyand

preventionofgastrointestinalinfections.Inanimalhus-

bandry,anumberofphagesthatattackonlyEHEC(ente-

rohemorrhagicE.coli)areusedagainstEHECinfections

Epidemiology Bacterialtyping.Strainsofabacterialspeciesareclassified

inphagovars(syn.lysotypes)basedontheirsensitivityto

typingbacteriophages.Recognitionofthebacterialstrain

responsibleforanepidemic,makingitpossibletofollow

upthe chainofinfectionandidentifytheinfection

sources.Thistypingmethodhasbeenestablishedfor

Salmonellatyphi,SalmonellaparatyphiB,Staphylococcus

aureus, Pseudomonasaeruginosa, andother bacteria,

althoughitisnowincreasinglybeingreplacedbynewmo-

lecularmethods,inparticularDNAtyping

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Kayser, Medical Microbiology © 2005 Thieme