31 Emerging Bacterial Foodborne Pathogens and Methods of Detection 715enterotoxin, Y-ST, which is involved in causing ill-
ness (Gugnani 1999, Takeda et al. 1999). This toxin
is a member of a family of heat-stable enterotoxins
produced by a number of bacteria such as E. coli,
Vibriospp., and Salmonellaspp. These toxins are
small peptides with a molecular weight of about 2
kDa. It is thought that Y-ST specifically causes diar-
rhea by binding to a membrane-bound guanylate
cyclase in the brush border membrane of the intes-
tine (Yoshino et al. 1995). This binding results in
increased levels of intracellular cGMP (cyclic
guanosine monophosphate), which stimulates chlo-
ride secretion and/or inhibits absorption. The end
result is net intestinal fluid secretion, and diarrhea
(Takeda et al. 1999).
The gene encoding Y-ST, ystA, has been found by
DNA-DNA hybridization in all pathogenic strains of
Y. enterocolitica, but not in any of the nonpatho-
genic strains (Takeda et al. 1999). This makes the
toxin and its gene a good candidate for detecting
specifically pathogenic strains of the bacteria.
LISTERIA MONOCYTOGENES
Listeria monocytogenesis a Gram-positive bacterium
responsible for the illness known as listeriosis. The
bacterium is widely distributed in the environment
and has been isolated from soil, plants, decaying veg-
etation, silage, water, and sewage.L. monocytogenes
is also commonly carried in the intestinal tract of
humans, as well as cattle, sheep, and goats (Donnelly
2001). Studies as of 1994 showed that 2–6% of peo-
ple in the United States carry the pathogen (Rocourt
1994). In food, it has been found in raw or processed
foods including dairy products, meat, vegetables, and
seafood (Gugnani 1999, Meng and Doyle 1997).
More specifically, outbreaks have been associated
with the consumption of ready-to-eat foods such as
coleslaw, milk (contaminated after pasteurization),
paté, pork tongue in jelly, and soft cheeses made from
raw milk (Altekruse et al. 1997, Donnelly 2001).
In the healthy individual, L. monocytogenesmay
cause meningitis, encephalitis, or septicemia. How-
ever, L. monocytogenesgenerally affects only a few
segments of society (Donnelly 2001), including the
elderly, immunosuppressed individuals, neonates, and
pregnant women (Kathariou 2002). In pregnant
women, the pathogen can cause bacteremia, which
if left untreated can lead to amnionitis and infection
of the fetus, resulting in stillbirth or premature birth
(Gugnani 1999). Less commonly, infection will re-
sult in cutaneous lesions and flu-like symptoms
(Meng and Doyle 1997). Although relatively few
people become ill with Listeriainfections (See
Table 31.1), there is a 20% mortality rate for those
that do. The result is that L. monocytogenesis re-
sponsible for 28% of food-related deaths in the
United States (Mead et al. 1999).
The pathogenic potential of L. monocytogeneshas
been known for decades, but its ability to be trans-
mitted through a food medium was discovered only
in 1981 (Meng and Doyle 1997). Listeriosis was
first diagnosed in a soldier with meningitis in World
War I (Rocourt 1994). It has also been postulated to
be the cause of Queen Anne’s 17 unsuccessful preg-
nancies in the 17th century (Saxbe Jr. 1972), due to
the pattern of miscarriages, the neonatal deaths ex-
perienced by 14 of the children, and the postulated
hydroencephaly experienced by her son, the Duke of
Gloucester, who survived until the age of 11.
L. monocytogenes is extremely robust and is
probably the most important pathogen able to grow
at refrigeration temperatures. It is able to grow at
temperatures as low as 2°C, and freezing food at
15°C or even repeatedly freezing and thawing
food has little effect on the viability of the organism
(Woteki and Kineman 2003). It can survive in a pH
range of 4.3 to 9.6, making it a contamination factor
in a number of cheeses and meats, as well as dairy
products (Donnelly 2001). The pathogen is also
resistant to desiccation. It can survive in the soil,
leaving it able to contaminate crops such as cabbage
or lettuce (Kathariou 2002). As well, the organism is
able to survive in salt concentrations of up to 25.5%,
which would cause the desiccation and death of
most bacteria (Donnelly 2001).
Listeriolysin (LLO) is the most important viru-
lence factor produced by L. monocytogenes(Jacobs
et al. 1999). LLO is a pore-forming protein of the
thiol-activated cytolysin family of toxins, which is
composed of 529 amino acids, encoded by the hlyA
gene (Jacobs et al. 1999). The gene is found within a
cluster of virulence genes found only in pathogenic
varieties of Listeria,and its expression is regulated
by PrfA, a global virulence transcriptional activator
protein (Jacobs et al. 1999).
On infection, a host macrophage will internalize
the Listeriacell through phagocytosis into its vac-
uole. The bacterium then secretes LLO, which binds
to cholesterol in the host membrane and forms pores
in the vacuole wall, allowing the bacterium to escape
into the cytoplasm. L. monocytogenescells then