Feb 11, 2016
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