Medical Microbiology

TheMorphologyandFineStructureofBacteria 151

Cytoplasm.............................................

Thecytoplasmcontainsalargenumberofsolutelow-andhigh-molecular-

weightsubstances,RNAandapproximately 20000 ribosomespercell.

Bacteriahave 7 0Sribosomescomprising30Sand50Ssubunits.Bacterial

ribosomesfunctionastheorganellesforproteinsynthesis.Thecytoplasm

isalsofrequentlyusedtostorereservesubstances(glycogendepots,poly-

merizedmetaphosphates,lipids).

TheMostImportantBacterialCytoplasmicMembraneProteins

Permeases Activetransportofnutrientsfromoutsidetoinsideagainsta

concentrationgradient.

Biosynthesis

enzymes

Requiredforbiosynthesisofthecellwall,e.g.,itsmurein(see

under“Cellwall”p. 1 52).Theenzymesthatcontributetothe

finalmureinbiosynthesisstepsareforthemostpartidentical

withthe“penicillin-bindingproteins”(PBPs).

Secretion

systemproteins

Foursecretionsystemsdifferinginstructureandmodeofaction

havebeendescribedtodate.Proteinsaremovedoutofthecell

withthehelpofthesesystems.Acommonfeatureofallfouris

theformationofproteincylindersthattraversethecytoplasmic

membraneand,inGram-negativebacteria,theoutercellwall

membraneaswell.Seep. 17 onthespecialrelevanceofthe

typeIIIsecretionsystemtovirulence.

Sensorproteins

(alsoknownas

signalproteins)

Transmitinformationfromthecell’senvironmentintoitsinte-

rior.Theso-calledreceiverdomainextendsoutward,thetrans-

mitterdomaininward.Thetransmissionactivityisregulatedby

thebindingofsignalmoleculestoareceivermodule.Intwo-

componentsystems,thetransmittermoduletransferstheinfor-

mationtoaregulatorprotein,activatingitsfunctionalmodule.

Thisregulatorsegmentcanthenbindtospecificgenesequences

andactivateordeactivateoneormoregenes(seealsoFig. 1. 4 ,

p. 1 9).

Respiratory

chainenzymes

Occurinbacteriawithaerobicmetabolism.Aerobicrespiration

functionsaccordingtothesameprinciplesascellularrespiration

ineurkaryotes.

TheCytoplasmicMembrane..........................

Thiselementarymembrane,alsoknownastheplasmamembrane,istypical

oflivingcells.Itisbasicallyadoublelayerofphospholipidswithnumerous

proteinsintegratedintoitsstructure.Themostimportantofthesemembrane

3

Kayser, Medical Microbiology © 2005 Thieme