Food poisoning and meat microbiology - Gracey's Meat Hygiene - page 261

effective control of the food chain. Ultimately, thorough
cooking will kill the organisms. The low infective dose
means that contamination unlikely to cause problems
when Salmonella spp. or Campylobacter spp. are present
may be enough to cause infection if E. coli O157
prevails.

Listeria monocytogenes
Of the several Listeria species, only L. monocytogenes is
thought to cause human infection. It is widely distrib-
uted in animals, birds, humans and the environment but
has only been recognised as a food-borne pathogen since
the 1980s. Like Yersinia spp., L. monocytogenes can grow
at a wide range of temperatures (0-42°C) and especially
well at 30-37°C. Listeria spp. can also grow slowly at
refrigeration temperatures. A wide range of serotypes
exist, but most human infection is associated with types
4 and 1/2.

Antimicrobial resistance

A particular concern with Salmonella spp. and E. coli is the
occurrence of antimicrobial resistance genes. Of particular
concern are the extended spectrum beta-lactamases (ESBL)
and/or AmpC-producing bacteria. ESBL are plasmid-encoded
enzymes found in Enterobacteriaceae that confer resistance
to a variety of beta-lactam antimicrobials, including
penicillins; second-, third- and fourth-generation
cephalosporins; and monobactams. Both ESBL and AmpC-
producing organisms have the potential to be transmitted
from animals to humans in food.

Infection in humans Most human cases are spo-
radic, and the extended incubation period (several
weeks) can make identification of the source very dif-
ficult. Unlike the case with most food-borne pathogens,
the illness produced is systemic rather than intestinal.
Most infected people are symptomless, and as many as
5% of the population are faecally excreting at any one
time. Illness is most likely to be seen in people with
reduced immunity, when the symptoms range from a
'flu-like' illness through fever and septicaemia. There is
a particular risk to pregnant women (who have a natu-
rally reduced immune capability) and the unborn child,
when the mortality can be as high as 30%. Although
there was a significant rise in reported cases of listeriosis
in the late 1980s, this was not maintained in subsequent
years. Listeriosis is now relatively uncommon, with
about 100 cases reported each year in the United
Kingdom.

Yersinia enterocolitica
Yersinia enterocolitica was identified as a human patho-
gen in the late 1930s. It is a Gram-negative non-spore-
forming bacterium which grows over a wide range of
temperatures (0-40°C) and optimally at 29°C. The range
of growth temperatures allows multiplication at refrig-
eration temperatures. It is widespread in the intestinal
tract of animals and is readily recovered from the envi-
ronment, including water and soil. It can be divided into
biotypes, serotypes and phage types. Different types are
associated with different parts of the world. In Europe,
serotype O3 is most commonly recorded in humans and
is also associated with pigs.

Source of human infection This is often not iden-
tified because of the length of the incubation period. The
organisms are ubiquitous and widespread in animals and
the environment. Listeriosis can cause animal disease,
including abortion in ewes and meningitis in younger
sheep. A wide range of foods have been implicated
including cheese, paté and poultry meat. Vulnerable
groups, especially pregnant women and the immuno-
compromised, are advised not to eat risk foods such as
patés and soft cheeses.

Infection in humans The incubation period is
about 3-7 days, and infection causes acute diarrhoea,
abdominal pain, fever and vomiting. It is more common
in children, although cases in adults may be followed by
longer-term problems including skin rashes and arthritis.
Infection is usually self-limiting, but in people with some
other underlying pathology, septicaemia with a high
mortality may follow. About 500 cases a year are reported
in the United Kingdom.

Clostridium perfringens
The mode of action of food poisoning with Clostridium
perfringens is through the consumption of large num-
bers of bacteria which rapidly form enterotoxin in the
small intestine. It is a Gram-positive spore former,
requiring anaerobic conditions for growth. In the United
Kingdom, a better understanding of food hygiene, in
particular the importance of thorough cooking and
rapid cooling, has resulted in a decline in the impor-
tance of this pathogen.

Source of human infection This organism is often
associated with pig meat products, either fresh or cured.
The ability to grow at refrigeration temperature means
that contaminated food, even if properly chilled, can
cause infection. Such foods can also cross-contaminate
other foods in the kitchen.