Medical Microbiology

ThePhysiologyofMetabolismandGrowthinBacteria 161

reactions,theenergyrequirementisconsumedintheformoflightorchem-

icalenergy—byphotosyntheticorchemosyntheticbacteria,respectively.

Catabolicreactionssupplybothenergyandthebasicstructuralelements

forsynthesisofspecificbacterialmolecules.Bacteriathatfeedoninorganic

nutrientsaresaidtobelithotrophic,thosethatfeedonorganicnutrients

areorganotrophic.

Human pathogenic bacteriaare always chemosynthetic, organo-

trophicbacteria(orchemo-organotrophs).

CatabolicReactions....................................

Organicnutrientsubstratesarecatabolizedinawidevarietyofenzymatic

processesthatcanbeschematicallydividedintofourphases:

Digestion.Bacterialexoenzymessplitupthenutrientsubstratesintosmaller

moleculesoutsidethecell.Theexoenzymesrepresentimportantpathogenic-

ityfactorsinsomecases.

Uptake.Nutrientscanbetakenupbymeansofpassivediffusionor,more

frequently,specificallybyactivetransportthroughthemembrane(s).Cyto-

plasmicmembranepermeasesplayanimportantroleintheseprocesses.

Preparationforoxidation.Splittingoffofcarboxylandaminogroups,phos-

phorylation,etc.

Oxidation.ThisprocessisdefinedastheremovalofelectronsandH+ions.

ThesubstancetowhichtheH 2 atomsaretransferrediscalledthehydrogen

acceptor.Thetwobasicformsofoxidationaredefinedbythefinalhydrogen

acceptor(Fig.3. 15 ).

&Respiration.Hereoxygenisthehydrogenacceptor.Inanaerobicrespira-

tion,theO 2 thatservesasthehydrogenacceptorisacomponentofaninor-

ganicsalt.

&Fermentation.Hereanorganiccompoundservesasthehydrogenaccep-

tor.

Themaindifferencebetweenfermentationandrespirationistheenergy

yield,whichcanbegreaterfromrespirationthanfromfermentationfora

givennutrientsubstratebyasmuchasafactorof 1 0.Fermentationprocesses

involvingmicroorganismsaredesignatedbythefinalproduct,e.g.,alcoholic

fermentation,butyricacidfermentation,etc.

Theenergyreleasedbyoxidationisstoredaschemicalenergyintheform

ofathioester(e.g.,acetyl-CoA)ororganicphosphates(e.g.,ATP).

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Kayser, Medical Microbiology © 2005 Thieme