The most commonly employed selective plating media used to enu-
merateC. perfringens employ antibiotic(s) as the selective agent and
sulfite reduction to produce black colonies as the differential reaction.
The most popular combinations are tryptose/sulfite/cycloserine (TSC)
medium and oleandomycin/polymyxin/sulfadiazine/perfringens (OPSP),
incubated anaerobically for 24 h at 37 1 C. A better diagnostic reaction is
obtained if pour plates are used since colonies on the agar surface of
spread plates can appear white.
Suspect colonies can be confirmed by the absence of motility, their
ability to reduce nitrate to nitrite, lactose fermentation, and gelatin
liquefaction. A traditional confirmatory test, the Nagler reaction, which
looks for the production ofa-toxin and its neutralization by antitoxin on
lactose egg-yolk agar is less favoured now because of its lower specificity
and the occurrence of lecithinase-negative strains.
Serotyping based on capsular antigens is employed for epidemiolog-
ical purposes. Most isolates from outbreaks can be serotyped and this
can be usefully supplemented by typing with bacteriocins, particularly
when serotyping is not possible.
A number of methods are available for the detection of enterotoxin.
Traditional biological tests such as the ligated ileal loop and mouse
challenge have been superseded by more sensitive, rapid, convenient and
humane serological techniques. A commercially available kit employs
reverse passive latex agglutination. The name derives from the fact that
in a standard agglutination assay, soluble antibody reacts with a partic-
ulate antigen such as bacterial cells. In a reversed passive latex aggluti-
nation assay, a soluble antigen reacts with antibody attached to latex
particles. These play no part in the reaction and are therefore passive, but
they do provide a visual signal when they cross-link as a result of the
antigen–antibody reaction.
7.6.5 Association with Foods
For an outbreak ofC. perfringensfood poisoning, the typical scenario
includes the following events:
(i) a meat dish containing spores ofC. perfringensis cooked;
(ii) the spores survive the cooking to find themselves in a genial
environment from which much of the competitive flora has been
removed;
(iii) after cooking, the product is subjected to temperature/time
abuse, such as slow cooling or prolonged storage at room
temperature. This allows the spores to germinate and multiply
rapidly to produce a large vegetative population;Chapter 7 213