Medical Microbiology

Enterobacteriaceae,Overview 279

significantserovarsbasedonO,H,andKantigens.Themostimportantpatho-

genicityfactorsofEnterobacteriaceaearecolonizingfactors,invasins,endo-

toxin,andvariousexotoxins.Enterobacteriaceaearethemostsignificantcon-

tributorstointestinalinfections,whichareamongthemostfrequentdiseases

ofallamongthedevelopingworldpopulace. &

Definitionandsignificance.TogetherwiththefamiliesVibrionaceaeand

others(p.224),theEnterobacteriaceaeformthegroupofGram-negative,fa-

cultativelyanaerobicrodbacteria.Theirnaturalhabitatistheintestinaltract

ofhumansandanimals.Somespeciescausecharacteristicdiseases.While

othersarefacultativelypathogenic,theyarestillamongthebacteriamost

frequentlyisolatedaspathogens(e.g.,E.coli).Theyareoftenresponsible

fornosocomialdiseases(seep.343ff.).

Taxonomy.ThetaxonomyoftheEnterobacteriaceae hasseenrepeated

changesinrecentdecadesandhasdoubtlessnotyetassumeditsfinal

form.Thefamilyincludes 41 generawithhundredsofspecies.Table4. 6 pro-

videsanoverviewofthemostimportantEnterobacteriaceaeinthefieldof

humanmedicine.

ThetaxonomicsystemappliedtoEnterobacteriaceaeisbasedonvarying

patternsofmetabolicprocesses(Fig.3. 36 ,p. 21 4).Oneoftheimportantchar-

acteristicsofthisbacterialfamilyislactosebreakdown(presenceofthelac

operon).ThelacoperonincludesthegeneslacZ(codesforb-galactosidase),

lacY(codesforb-galactosidepermease),andlacA(codesfortransacetylase).

Lactose-positiveEnterobacteriaceaearegroupedtogetherascoliformEnter-

obacteriaceae.Salmonellaeandmostoftheshigellaearelactose-negative.

Morphologyandculture.EnterobacteriaceaeareshortGram-negativerods

withroundedends,0.5– 1 .5lmthick,and 24 lmlong(Fig.4. 17 a).Many

haveperitrichousflagellation.Specieswithmanyflagella(e.g.,Proteusspe-

cies)showmotilityontheagarsurface,whichphenomenonisknownas

“swarming.”SomeEnterobacteriaceaepossessacapsule.

Allbacteriainthisfamilycanreadilybeculturedonsimplenutrientme-

diums.Theyarerapidlygrowingfacultativeanaerobes.Theirmeangenera-

tiontimeinvitrois 20 – 30 minutes.Theyshowresistancetovariouschemicals

(bilesalts,crystalviolet),whichfactismadeuseofinselectiveculturing.Endo

agarisanimportantselectiveindicatormedium;itallowsonlyGram-nega-

tiverodbacteriatogrowandindicateslactosebreakdown(Fig.4. 17 b).

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Kayser, Medical Microbiology © 2005 Thieme