ETEC is also frequently associated with so-called traveller’s diarrhoea.
However since 1982, enterohaemorrhagic E. coli (EHEC) particularly
associated with serotype O157:H7 has been recognized as the cause of a
number of outbreaks of haemorrhagic colitis and haemolytic uraemic
syndrome, particularly in North America, where foods such as under-
cooked ground meat, raw milk and fresh produce have been implicated.
An exponential rise in isolations of O157:H7 was reported in Canada
between 1982 and 1986 and a study in the UK between 1985 and 1988
suggested that the increased reporting of isolations there (118 in England
and Wales and 86 in Scotland) represented a real increase. The number of
cases in the UK continued to increase until 1997 and has fluctuated
between 600 and 1000 isolations per year since then. Other European
countries have also reported increased isolation rates.
Two further types ofE. coliare recognized as causes of diarrhoea,
primarily in children. Termed enteroaggregativeE. coli(EaggEC) and
diffusely adherentE. coli(DAEC), they have characteristic patterns of
adherence to Hep-2 cells in culture. The emergence of these numerous
pathotypes ofE. coliis thought to reflect the plasticity of the organism’s
genome. The acquisition, loss or rearrangement of genetic elements
introduces new pathogenicity and virulence characterisitics and the differ-
ent pathotypes represent strains sharing common virulence determinants.
7.8.2 The Organism and its Characteristics
Escherichiais the type genus of the Enterobacteriaceae family andE. coli
is the type species of the genus. It is a catalase-positive, oxidase-negative,
fermentative, short, Gram-negative, non-sporing rod. Genetically,E. coli
is very closely related to the genusShigella, although characteristically it
ferments the sugar lactose and is otherwise far more active biochemically
thanShigellaspp. Late lactose fermenting, non-motile, biochemically
inert strains of E. coli can however be difficult to distinguish from
Shigella.
E. coli can be differentiated from other members of the Enter-
obacteriaceae on the basis of a number of sugar-fermentation and other
biochemical tests. Classically an important group of tests used for this
purpose are known by the acronym IMViC. These tested for the ability
to produce:
(i) indole from tryptophan (I);
(ii) sufficient acid to reduce the medium pH below 4.4, the break
point of the indicator methyl red (M);
(iii) acetoin (acetylmethyl carbinol) (V); and
(iv) the ability to utilise citrate (C).Chapter 7 217