in 28S rRNA thus stopping protein synthesis in the cell. Stx 2 also
comprises an A and B subunit but these are larger than in Stx 1 (Mr
35 kDa and 10.7 kDa respectively) and do not cross-react immunologi-
cally, though they do share a 60% amino acid sequence homology, with
Shiga toxin. Both toxins have been shown to be phage encoded in a
number of strains.
7.8.4 Isolation and Identification
Selective techniques forE. colimostly exploit the organism’s tolerance of
bile and other surfactive compounds, a consequence of its natural
habitat, the gut. Aniline dyes and the ability of many strains to grow
at temperatures around 44 1 C are also used as selective agents.
The first selective and differential medium was that originally devised
by MacConkey in 1905. It has been variously modified since but its
essential characteristics have remained unchanged. Bile salts (and some-
times the aniline dye, crystal violet) act as inhibitors of Gram-positive
and some fastidious Gram-negative bacteria. Lactose is included as a
fermentable carbohydrate with a pH indicator, usually neutral red.
Strong acid producers like Escherichia, Klebsiella, and Enterobacter
produce red colonies, non-lactose fermenters such as Salmonella,
Proteus, and Edwardsiella, with rare exceptions produce colourless
colonies. MacConkey agar is not however strongly selective and will
support the growth of a number of non-Enterobacteriaceae including
Gram-positives such as enterococci and staphylococci.
Eosin/methylene blue agar is a popular selective and differential
medium in North America. The aniline dyes eosin and methylene blue
are the selective agents but also serve as an indicator for lactose
fermentation by forming a precipitate at low pH. Strong lactose ferment-
ers produce green-black colonies with a metallic sheen.
A biochemical feature ofE. coliincreasingly being used in diagnostic
media isb-glucuronidase activity, which is possessed by around 95% of
E. colistrains but by only a limited number of other bacteria. A fluorogenic
or chromogenic glucuronide is incorporated into a conventional medium
and enzyme activity detected by the production of colour or fluorescence.
Most widely used is the fluorogen 4-methylumbelliferyl-b-D-glucuronide
(MUG) which is hydrolysed to produce fluorescent 4-methylumbelliferone.
Suspect colonies from selective and differential media can be con-
firmed by further biochemical testing.
Detection ofE. coliO157:H7 is based on phenotypic differences from
most other serotypes: its inability to ferment sorbitol on MacConkey
sorbitol agar and absence ofb-glucuronidase activity in most strains.
PresumptiveE. coliO157:H7 from these tests must then be confirmed
serologically for which a latex agglutination kit is commercially available.
222 Bacterial Agents of Foodborne Illness