inconsistent with a foodborne intoxication. Finally, the ability to pro-
duce the toxin is not confined to pathogenicY. enterocolitica, but has
also been demonstrated in numerous environmental strains and a
number of otherYersiniaspecies as well.
7.16.4 Isolation and Identification
A large number of procedures for the isolation and detection of
Y. enterocoliticahave been developed. Enrichment procedures usually
exploit the psychrotrophic character of the organism by incubating at
low temperature, but this has the disadvantage of being slow with the
attendant possibility of overgrowth by other psychrotrophs present.
Some workers have included selective agents in their enrichment media
but some strains, such as serotype O8, are reported to be sensitive to
selective agents. The most commonly used enrichment media are phos-
phate buffered saline (PBS) or tryptone soya broth (TSB) most usually
incubated at 4 1 C for 21 days.Y. enterocoliticaand related species are
more alkali resistant than many other bacteria so the pH of enrichment
media is sometimes adjusted to 8.0–8.3 or cultures subjected to a short
post-enrichment alkali treatment.
The best results for the selective isolation ofY. enterocoliticafrom
foods and enrichment broths have been obtained with cefsulodin/ irga-
san/novobiocin (CIN) agar. In addition to the antibiotics, the medium
contains deoxycholate and crystal violet as selective agents and mannitol
as a fermentable carbon source. After incubation at 28 1 C for 24 h,
typical colonies ofY. enterocoliticaappear with a dark-red centre sur-
rounded by a transparent border. Isolates can be confirmed and biotyped
by biochemical tests.
In vitrotests to distinguish between environmental and pathogenic
strains ofY. enterocoliticahave been referred to above (Section 7.16.2).
Techniques using gene probes to detect the virulence-associated plasmid
by a colony hybridization test have also been used with some success and
offer the possibility of detecting potentially pathogenic strains in foods
without the need for lengthy enrichment procedures.
7.16.5 Association with Foods
Pigs are recognized as chronic carriers of thoseY. enterocoliticasero-
types most commonly involved in human infections (O3, O5, 27, O8,
O9). The organism can be isolated most frequently from the tongue,
tonsils and, in the gut, the caecum of otherwise apparently healthy
animals. Despite this, pork has only occasionally been shown to be the
vehicle for yersiniosis, although a case control study in Belgium, which
has the highest incidence of yersiniosis, implicated a national prediliction
266 Bacterial Agents of Foodborne Illness