organism grows slowly and cultures are normally incubated for three
weeks before they are considered negative. In view of this, testing foods
for the organism is not practically feasible or useful. Cattle are tested for
the presence of antibodies to the organism in the ‘Ring Test’. Stained
antigen is mixed with the test milk, if antibodies toBrucellaare present
(indicative of infection) then they will cause the antigen to clump and rise
with the milkfat on standing to form an intense blue-violet ring at the top
of the milk.
7.3.5 Association with Foods
Although brucellosis has sometimes been associated with the consump-
tion of inadequately cooked meat from an infected animal, raw milk or
cream are the principal food vehicles.Brucellais readily killed by normal
milk pasteurization conditions so there is no risk from pasteurized milk
or products made from it. Cheeses made from unpasteurized milk can
sometimes pose a problem since the organism can survive the cheese-
making processes and subsequent storage in the product.
7.4 Campylobacter
7.4.1 Introduction
Campylobacterhas been known as a veterinary problem since the early
years of the 20th Century when the original isolate, known then asVibrio
fetus, was associated with infectious abortion in sheep and cattle. In 1931
the speciesVibrio jejuniwas described as the cause of winter dysentery in
calves and in 1946 a similar organism was isolated from blood cultures of
patients in a milk-borne outbreak of acute diarrhoea. Later King,
working with human blood isolates, distinguished two groups on the
basis of their optimum growth temperature. One group corresponded to
Vibrio fetusand the second, ‘thermophilic’, group, which grew best at
421 C, came from patients with preceding diarrhoea.
Both groups differ from the cholera and halophilic vibrios biochem-
ically, serologically and in their mol% GþC ratio, and were reclassified
into the new genusCampylobacterin 1963.
In the 1970s, with the development of suitable selective media, it was
established thatCampylobacter jejuni, and to a lesser extentCampylobacter
coli, are a major cause of diarrhoeal illness, rivalling and even surpassing
Salmonella in importance in many countries. Campylobacter laridis,
C. concisus and C. hyointestinalis have also been isolated occasion-
ally from patients with diarrhoea and C. pylori, now reclassified as
Helicobacter pylori, has been associated with gastritis and stomach and
duodenal ulcers.
192 Bacterial Agents of Foodborne Illness