74 Chapter 3
Listeria monocytogenes. Listeria mono-
cytogenes was initially described as a dairy
pathogen, but now it is well known that L.
monocytogenes is an environmental microor-
ganism that associates with the cool, damp
environments that are associated with many
food processing establishments. Although
not as frequent as many other foodborne dis-
eases, L. monocytogenes has the second
highest fatality rate (21%) and highest hospi-
talization rate (90%) of all foodborne patho-
gens (CDCP, 2001 ).
A considerable number of human listerio-
sis outbreaks are attributable to cheese sources
(Table 3.3 ). Many of these occurrences have
involved considerable morbidity and mortal-
ity. Over the period from 1983 to 1987 in
Switzerland, Vacherin Mort d ’ Or cheese was
the cause of 122 cases and 33 deaths from lis-
teriosis. Two different epidemic - associated
strains of L. monocytogenes serotype 4b
were isolated from case patients, unconsumed
cheese, wooden shelves, and brushes used
in cheese - ripening cellars. The high levels of
L. monocytogenes detected (10^4 to 10^6 cfu/g)
on cheese surface samples indicated contami-
nation and growth of the organism on the
cheese during ripening.
Between January and August 1985, 152
cases of listeriosis ( L. monocytogenes sero-
type 4b) were reported in Los Angeles County,
CA. The overall case fatality rate was 33%,
of which 65% were prenatal. Mexican - style
soft cheese was epidemiologically and bacte-
riologically associated with the occurrence of
the disease. A follow - up investigation of this
outbreak from a pasteurized milk cheese
revealed gross environmental and equipment
contamination as well as inadequate pasteuri-
zation. The widespread distribution of L.
monocytogenes in the environment and its
ability to survive on dry and moist surfaces
favors post - processing contamination of dairy
products from both raw milk and factory sites
(McLauchlin, 1997 ). Most studies have indi-
cated that L. monocytogenes is not suffi ciently
heat resistant to survive pasteurization ofassociated with foodborne disease cases.
High temperature cooking is also applied to
some fresh cheeses, such as cottage cheese,
in which temperatures of 52 ° C to 60 ° C for
1.5 to 2 hours is believed suffi cient to kill
coliforms and psychrotrophic bacteria (Fox
et al., 2000 ).
Cooking temperatures, pH/acidity, and
moisture content of cheeses, together with
the length of maturation of the fi nished
product and the fact that they are stored at a
controlled temperature, constitute a hurdle
system of preservation that acts as a series of
control steps to inhibit the growth of patho-
genic bacteria. Fresh cheeses, for example
cream cheese, cottage cheese, quarg, or
ricotta, are unripened cheeses, ready for con-
sumption immediately after production. Such
cheeses, which make a signifi cant contribu-
tion to the ingredients sector, rely on heat
treatment of cheese milk to produce a safe
product because no ripening occurs.
Little et al. (2008) have identifi ed other
contributory factors likely to cause problems
with the microbiological quality of cheese
made from raw, thermized, or pasteurized
milk. Unsatisfactory quality of cheeses was
more frequently associated with premises
having poor management and control sys-
tems. Appropriate hygiene measures to avoid
contamination from the production environ-
ment and appropriate temperature control of
cheeses are critical for minimizing contami-
nation with and growth of pathogens.
The major pathogens of concern in cheeses
are those considered for raw and pasteurized
milk. While a number of the bacterial patho-
gens have been responsible for major and
sporadic outbreaks of disease, others have
not resulted in cheese - related outbreaks but
nevertheless have zoonotic potential through
evidence of their occurrence and survival
during the manufacturing and ripening pro-
cesses. Outbreaks, prevalence, and challenge
studies associated with cheese are described
here for the major pathogens recorded in the
literature to date.