1. Escherichia coli is a straight gram-negative short rod/bacilli, which is an important member of the family Enterobacteriaceae.
  2. E. coli is the most abundant facultative anaerobe in the colon and feces.
  3. Escherichia coli cells are small rods 1.0-2.0 micrometers long, with a radius of about 0.5 micrometers. However, the size varies with the medium, and faster-growing cells are larger.
  4. The generation (doubling) time of Escherichia coli is 20 minutes.
  5. E.coli are lactose fermenters that give pink colonies in MacConkey agar (this property distinguishes E.coli from Salmonella and Shigella-two most common intestinal pathogens)
Morphology of E.coli on different medias.
  1. Antigenic properties: There are more than 1000 antigenic types of Escherichia coli. 
    a. O-cell wall antigens (>150 types)
    b. H- flagellar antigen (>50 types)
    c. K- capsular antigen (>90 types)
Catalase testPositive
Oxidase testNegative
Nitrate reduction testPositive
Methyl-Red (MR) testPositive
Voges-Proskauer (VP) testNegative
Citrate utilization testNegative
Acetate utilization testPositive
Indole testPositive
Pyrrolidonyl-β-naphthylamide (PYR) testNegative
H2S production testNo
Urease testNegative
Oxidative-fermentative (OF) testFermentative
MUG testPositive
TSI reactionsAcid/Acid, Gas,
Phenyl Pyruvic acid (PPA) testNegative
Lysine decarboxylation test+
Arginine decarboxylation test-/+ (strain variability)
Ornithine decarboxylation test+/- (strain variability)
Sugar fermentation test