steaks can often be consumed safely when the interior is undercooked,
this is because microbial contamination is usually a surface phenom-
enon. Comminution of the meat, however, will mix surface contaminants
into the middle of products and they will therefore need cooking
throughout to ensure microbial safety. The USDA has produced regula-
tions specifying that the centre of beef hamburgers should reach on
cooking: 71.1 1 C (160 1 F) instantaneously for consumers and 68.3 1 C
(155 1 F) for 16 seconds in food service operations.
Outbreaks of EHEC have been reported with other foods. Lettuce
has been associated on several occasions and unpasteurised apple
juice was the vehicle in a large outbreak in the US. In the summer of
1996, an epidemic in Japan involved over 9000 cases and 12 deaths
in children. The largest outbreak during the epidemic, in Sakai City,
involved 5700 people and was associated with contaminated radish
sprouts, and the same vehicle was implicated in a further outbreak
the following year. Alfalfa sprouts were also implicated in an outbreak in
the US.
Outbreaks caused by acidic foods such as apple juice and fermented
sausages, and laboratory studies with mayonnaise, remind us of the
potential for bacteria to survive for prolonged periods at pH values that
do not permit growth, particularly when the product is refrigerated.
EHEC does appear to have a more marked ability to survive at low pH
values than some other bacteria and this may also account for the
relatively low infectious dose, 2–2000 cells, recorded in outbreaks.
7.9 Listeria monocytogenes
7.9.1 Introduction
L. monocytogenes is the only important human pathogen among the
six species currently recognized within the genus Listeria, although
L. seeligeri,L. welshimeri, andL. ivanoviihave occasionally been asso-
ciated with human illness. It was first described by Murray in 1926 as
Bacterium monocytogenes, the cause of an infection of laboratory rabbits
where it was associated with peripheral blood monocytes as an intra-
cellular pathogen, and it has since been established as both an animal
and human pathogen. As an important veterinary problem, it causes two
main forms of disease: a meningoencephalitis most common in adult
ruminants such as sheep and cattle, and a visceral form more common in
monogastrics and young ruminants which attacks organs other than the
brain causing stillbirth, abortion and septicaemia. Listeriosis in sheep
increased in Britain from 86 recorded incidents in 1979 to 423 in 1988.
This was partly due to the increased size of the national flock over that
period but has also been attributed to changes in silage-making
224 Bacterial Agents of Foodborne Illness