Medical Microbiology

Bacteriophages 185

&Penetration.Injectionofthephagegenome.Enzymaticpenetrationofthe

wallbythetailtubetipandinjectionofthenucleicacidthroughthetailtube.

&Reproduction.Beginningwithsynthesisofearlyproteins(zerototwo

minutesafterinjection),e.g.,thephage-specificreplicasethatinitiatesrepli-

cationofthephagegenome.Thenfollowstranscriptionofthelategenesthat

codeforthestructuralproteinsoftheheadandtail.Thenewphageparticles

areassembledinamaturationprocesstowardtheendofthereproduction

cycle.

&Release.Thisstepusuallyfollowsthelysisofthehostcellwiththehelpof

mureinhydrolasecodedbyaphagegenethatdestroysthecellwall(Fig.3. 27 ).

Dependingonthephagespeciesandmilieuconditions,aphagereproduction

cycletakesfrom 20 to 60 minutes.Thisiscalledthelatencyperiod,andcan

beconsideredasanalogoustothegenerationtimeofbacteria.Dependingon

thephagespecies,aninfectedcellreleasesfrom 20 toseveralhundrednew

phages,whichnumberdefinestheburstsize.Thusphagesreproducemore

rapidlythanbacteria.Inviewofthisfact,onemightwonderhowanybacteria

havesurvivedinnatureatall.Itisimportantnottoforgetthatcellpopulation

densityisamajorfactordeterminingtheprobabilityoffindingahostcellin

thefirstplaceandthatsuchdensitiesarerelativelysmallinnature.Another

aspectisthatonlyasmallproportionofphagesreproducesolelybymeansof

theselyticorvegetativeprocesses.Mostaretemperatephagesthatlysogen-

izetheinfectedhostcells.

ReleaseofPhagesfromtheHostCell

Fig.3. 27 Attheendofthe

phage maturation process,

thehostcellislysedtore-

leasethenewphages.Lysis

occursbyaphage-encoded

murein hydrolase, which

gainsaccesstothemurein

throughmembranechannels

formed by the phage-en-

codedproteinholin.

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Kayser, Medical Microbiology © 2005 Thieme