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Enterobactericae
Large group of grame negative bacilli,Non spore forming
General characters of Enterobacteriaceae
1- They are facultative anaerobes.
2- They all ferment glucose (fermentation of other sugars varies).
3- They are oxidase negative.
4- They reduce nitrates to nitrites
Classification of Enterobaceriaceae
According to Lactose fermentation, by (Cultivation on macconkey’s agar) into
Lactose fermenters (A
(produce pink colonies )
E.coli, Klebsiella, Citrobacter and Enterobacter.
They are called coliforms
B) Lactose non-fermenters
(produce pale yellow colonies)
Salmonella, shigella, Yersinia and proteus.

MacConkey agar
Selective and differential
Main components
Pepton H2O
Bile salts
Lactose
neutral red indicator

Principle
Lactose fermenting strains grow as pink. The pink color is due to production of acid from lactose, absorption of neutral red and a subsequent color change of the dye .

Lactose non-fermenting strains, such as Shigella and Salmonella are colorless and transparent and typically do not alter appearance of the medium.
Escherichia coli
E. coli is the most common and important member of the genus Escherichia
Habitat: Natural habitat is the intestinal tract of humans and animals (part of the normal flora).
Morphology: gram negative bacilli, motile, some strains are capsulated.
Antigenic structure: They posses O (somatic) antigen, H (flagella) Ag, some possess K (capsular) Ag

Virulence Factors of Escherichia coli
(A) Surface virulence factors
1.Pili: mediates adherence to mucosal surfaces.
2-K or capsular polysaccharide Ag which is antiphagocytic
3-LPS: responsible for the endotoxin manifestations.
(B) Toxins:
4-Two exotoxins (they are enterotoxins) one is heat labile (LT) and the second is heat stable (ST). Produced by Enterotoxigenic strains of E .coli.
5-Verotoxin or shiga toxin: Produced by Enterohaemorrhagic strains of E.coli
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