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b-glucuronidase activity; an enzyme possessed by most, but not all,
strains ofE. coliand relatively uncommon in other bacteria.
One criticism of using coliforms and faecal coliforms is that their
absence could give a false reassurance of safety when lactose-negative
organisms predominate. The lactose-negative organisms include, not
onlySalmonellaandShigella, but also enteroinvasive strains ofE. coli
(EIEC) such as O124. For this reason, tests for the whole of the
Enterobacteriaceae are increasingly being used. The Enterobacteriaceae
includes even more genera of non-faecal origin than the coliforms, such
asErwiniaandSerratiawhich are predominantly plant associated. For
this reason Enterobacteriaceae counts are used more generally as an
indicator of hygienic quality rather than of faecal contamination and
therefore say more about general microbiological quality than possible
health risks posed by the product. For instance, the presence of high
numbers of Enterobacteriaceae in a pasteurized food would be cause
for concern although it would not necessarily imply faecal contami-
nation, and one would expect to find Enterobacteriaceae on fresh
vegetables without the product necessarily being hazardous. The poten-
tial significance of genera of the Enterobacteriaceae is summarized in
Table 10.1.
Some food microbiologists have tried to distinguish between ‘indica-
tor’ organisms, which relate to general microbiological quality, and
so-called ‘index’ organisms, which suggest that pathogens may be
present. As will be apparent from the discussion above, this is not a
simple distinction to make and the terminology has not been widely
adopted.

Table 10.1 Significance of genera of the Enterobacteriaceae in the monitoring of
foods

Genus

Predominantly

faecal origin

Usually detected

in ‘coliform tests’

Typically enteropathogenic

in humans

Citrobacter No Yes No
Edwardsiella Yes No No
Enterobacter No Yes No
Erwinia No No No
Escherichia Yes Yes No
Hafnia No No No
Klebsiella No Yes No
Proteus No No No
Salmonella Yes No Yes
Serratia No No No
Shigella Yes No Yes
Yersinia Yes No No

These comments are generalizations and there are exceptions to most of them. Adapted from
‘Micro-organisms in Foods 1: Their Significance and Methods of Enumeration’. 2nd Edition
ICMSF, University of Toronto Press, 1978

372 Methods for the Microbiological Examination of Foods