campylobacters possess catalase and superoxide dismutase, the accumu-
lation of peroxides and superoxide in media during storage or incubation
can inhibit growth. For this reason an incubation atmosphere of 5–6%
oxygen with about 10% carbon dioxide and media containing oxygen
scavenging compounds such as blood, pyruvate, ferrous salts, charcoal
and metabisulfite are commonly used.
To isolate the relatively low numbers of cells that may be present in
foods, several selective enrichment media are used which include cock-
tails of antibiotics such as polymyxin B, trimethoprim and others as
selective agents. In many cases cells isolated from food or other envi-
ronmental sources have been sub-lethally injured as a result of stresses
such as freezing, drying or heating and, as a result, are more sensitive to
antibiotics and toxic oxygen derivatives. This can mean that they will not
grow on the usual selective media unless allowed a period for recovery
and repair in which case a resuscitation stage of 4 h at 37 1 C in a non-
selective environment is recommended.
After selective enrichment for 24 and 48 h under microaerobic condi-
tions at 42–43 1 C, samples are streaked on to selective plating media.
These normally contain a nutrient-rich basal medium supplemented with
oxygen scavengers such as blood and/or FBP (a mixture of ferrous
sulfate, sodium metabisulfite, and sodium pyruvate), and a cocktail of
antibiotics similar to those used for selective enrichment. It is important
to store pre-prepared media under nitrogen, at 4 1 C and away from light
to reduce the build-up of toxic oxides.
Colonies are non-haemolytic and have a rather unimpressive flat,
watery appearance with an irregular edge and a grey or light-brown
coloration. Suspect colonies are examined microscopically for motility
and morphology and subjected to a range of tests after purification.
C. jejuni,C. coli,andC. laridisare catalase and oxidase positive, reduce
nitrate to nitrite, grow at 42 1 C but not at 25 1 C microaerobically and
cannot grow aerobically at 37 1 C.C. laridisis resistant to nalidixic acid
whileC. jejuniandC. coliare not.C. jejuniandC. colican be distinguished
by the ability of the former to hydrolyse hippurate. Various typing
schemes have been proposed for epidemiological investigations. Biotyping
based on biochemical tests and the more discriminating serotyping
schemes of Lior and Penner have been used most frequently but not
routinely. Molecular methods such as ribotyping and flagellin gene typing
have been used but pulsed-field gel electophoresis has become the sub-
typing method of choice due to its sensitivity and discriminatory power.
7.4.5 Association with Foods
The incidence of Campylobacter infection is characterized by large
numbers of sporadic cases rather than single source outbreaks. Infection
196 Bacterial Agents of Foodborne Illness