Since production of mixed acids and acetoin are alternative pathways for
the metabolism of pyruvate, most species of the Enterobacteriaceae are
either VP positive or methyl red positive. In the IMViC tests, most
strains ofE. coliare indole and methyl red positive and VP and citrate
negative (Table 7.7). The tests are still used for identification purposes
but nowadays usually as part of the larger range of tests available in
modern miniaturized test systems.
E. coliis a typical mesophile growing from 7–10 1 Cupto50 1 C with an
optimum around 37 1 C, although there have been reports of some ETEC
strains growing at temperatures as low as 4 1 C. It shows no marked heat
resistance, with a D value at 60 1 C of the order of 0.1 min, and can
survive refrigerated or frozen storage for extended periods. A near-
neutral pH is optimal for growth but growth is possible down to pH 4.4
under otherwise optimal conditions. The minimumawfor growth is 0.95.
A serotyping scheme forE. colibased on lipopolysaccharide somatic O,
flagellar H, and polysaccharide, capsular K antigens was proposed by
Kauffman in the 1940s. As currently applied in the O:H system, principal
serogroups are defined by O antigens and then subdivided into serotype on
the basis of H antigens. Strains of each category of pathogenicE. colitend
to fall within certain O:H serotypes, so the scheme plays an important role
in detecting pathogens as well as in epidemiological investigations.
7.8.3 Pathogenesis and Clinical Features
There are four major categories of diarrhoeagenic E. coli based on
distinct, virulence properties.
7.8.3.1 Enterotoxigenic E. coli (ETEC). Illness caused by ETEC
usually occurs between 12 and 36 h after ingestion of the organism.
Symptoms can range from a mild afebrile diarrhoea to a severe cholera-
like syndrome of watery stools without blood or mucus, stomach pains
and vomiting. The illness is usually self-limiting, persisting for 2–3 days,
although in developing countries it is a common cause of infantile
diarrhoea where it can cause serious dehydration.
Table 7.7 The IMViC tests
Indole Methyl Red Voges Proskauer CitrateEscherichia coli þþ
Shigella V þ
SalmonellaTyphimurium þþ
Citrobacter freundii þþ
Klebsiella pneumoniae þþ
Enterobacter aerogenes þþ
218 Bacterial Agents of Foodborne Illness