General characters: Predominant aerobic bacterial flora of

large intestine. Composed of non sporing, nonacid fast,

gram negative bacilli. May or may not be capsulated. Motile by peritrichate flagella or non

motile. Grow readily in ordinary media. Ferments glucose, producing acid and gas

or acid only.

Reduces nitrates to nitrites. Form catalase but not oxidase.

CLASSIFICATION:

TRIBE 1: Escherchiae Genus : 1.Escherchia 2.Edwardsiella 3.Citrobacter 4.Salmonella 5.Shigella

TRIBE 2: Klebsiellae Genus : 1.Klebsiella 2.Enterobacter 3.Hafnia 4.Serratia

TRIBE 3: Proteae Genus : 1.Proteus 2.Morganella 3.Providencia

TRIBE 4: Erwinieae Genus : Erwinia

ESCHERICHIA COLI Parasite living only in human or

animal intestine

Voided in faeces and remain viable for some days only

Detection of E.coli in drinking water – recent pollution with or human or animal faeces

MORPHOLOGY:

Gram negative, straight rods size 1-3x0.4-0.7 microns Arrangement-singles/pairs Motility-Peritrichious flagella some

are non motile Some strains are Capsulated and

Fimbriated Non sporing

CULTURAL CHARACTERISTICS:

Aerobe and facultative anaerobe Temp.range 10-40c optimum :37c Good growth occurs on ordinary media Growth inhibited on selective media-

DCA/SS agar In broth turbid growth occurs

COLONY MORHOLOGY:

2 Types of colonies are observed1.Smooth colonies: Large,thick,greyish white,

moist,smooth or partially translucent discs.

2.Rough colonies: colonies are irregular,dull surface and are often auto agglutinable.

ON BLOOD AGAR: Hemolytic colonies seen.

ON MAC CONKEY AGAR: Bright pink colonies seen due to lactose fermentation.

BIOCHEMICAL REACTIONS: Catalase : positive Oxidase : negative Indole : positive Methylred: positive VP : negative Citrate : negative

SUGAR FERMENTATION TESTS: Glucose : positive Lactose : positive Mannitol: positive Maltose : positive

Gelatin liquefaction : negative H2s production test : negative Urease test : negative

Growth does not occur in KCN media

ANTIGENIC STRUCTURE:

Serotyping done based on 3 antigens 1.SOMATIC antigen “O” 2.CAPSULAR antigen”K” 3.FLAGELLAR antigen”H”

“K” ANTIGEN: Acid polysaccharide antigen located in

envelope Encloses “O” Ag Contributes virulence by inhibiting

phagocytosis Thermolabile ‘L’ Ag “K” Ag ‘A’ Ag Thermostabile ‘B’ Ag