716 Part VII: Food Safety
replicate and use the host’s actin molecules to trans-
port themselves to the host cell’s plasma membrane
where they force the host cell to extend a pseudopod
to an adjacent cell that engulfs the pseudopod. The
L. monocytogenesagain secretes LLO to escape the
double-membrane vesicle and starts the cycle over
again (Kathariou 2002).
LLO is secreted as a 58-kDa protein (Jacobs et al.
1999). It binds as a monomer to the membrane sur-
face of the target, with subsequent oligomerization
into large arc- or ring-shaped structures that punc-
ture the membrane (Alouf 1999). They form pores
in the membrane that are about 20 nm in diameter
(Jacobs et al. 1999).
The optimal pH for LLO activity is pH 5.6
(Stachowiak and Bielecki 2001). This means that
LLO secreted within the acidic phagosome is active,
thus allowing pore formation and escape of the cells
to the cytoplasm. The cytoplasm, however, has a
more neutral pH. This renders the LLO inactive and
thus unable to lyse the cell during the bacterium’s
replicative phase (Beauregard et al. 1997). A num-
ber of other external stimuli such as growth temper-
ature and sugar availability also affect the expres-
sion and secretion of LLO (Jacobs et al. 1999,
Kathariou 2002).
CLOSTRIDIUM PERFRINGENS
C. perfringensis an anaerobic Gram-positive, spore-
forming bacterium that commonly inhabits soil,
water, sewage, and the intestinal tract of a number of
warm-blooded animals, including humans. The path-
ogen is responsible for cases of human gas gangrene
and two different foodborne diseases, a mild diar-
rhea (Type A) and a rare human necrotic enteritis
(Type C). Type A diarrhea has become one of the
most common foodborne diseases of the industrial-
ized world, often isolated from samples taken from
restaurants, hospitals, and old age homes (Brynstad
and Granum 2002). C. perfringensType A disease
generally occurs 9–17 hours after ingesting the con-
taminated food, with full recovery occurring within
12–24 hours (Boyd 1988). The type C disease has an
incubation time of 5–6 hours followed by an acute
sudden onset of severe abdominal pain, diarrhea,
vomiting, and necrotic inflammation of the intestine.
Death results in 15–25% of those infected (Brynstad
and Granum 2002).
Illness generally occurs after eating improperly
canned foods. C. perfringensis able to grow at any
temperature between 15 and 50°C, with an optimum
of 45°C (Brynstad and Granum 2002). Unless food
is fully sterilized, only the vegetative cells are killed
by heat treatment, leaving live C. perfringensspores
(Woteki and Kineman 2003). The spores survive in
an inactive state until conditions are favorable for
vegetative growth to resume (Brynstad and Granum
2002).
C. perfringens produces a number of toxins,
including -toxin, phospholipase C, and perfring-
olysin O (PLO) (Bryant 2003). In fact, members of
the species are able to produce a total of 13 different
toxins, although each individual cell does not nor-
mally produce all of them (Brynstad and Granum
2002).
In the Western world, the C. perfringensentero-
toxin (CPE) is the most important disease-causing
toxin. CPE is produced in sporulating C. perfrin-
gens, causing the Type A disease, and is thought to
be one of the main causes of diarrhea. CPE is a two-
domain protein that binds to claudin proteins found
in the tight junctions between intestinal cells. It then
causes pore formation in the cells, leading to the loss
of cations into the host’s intestinal space, resulting
in water loss and diarrhea (Brynstad and Granum
2002). Since many healthy people have high levels
of C. perfringensin their feces, it is essential that
any test detecting the bacteria during an outbreak
situation detect the CPE toxin in both the food and
the infected individual to prove the causality of the
infection (Brynstad and Granum 2002).
Perfringolysin O (PLO) is found in the Type C
human necrotic enteritis–causing C. perfringens. It
is a thiol-activated cytolysin, similar to LLO from L.
monocytogenes. Like LLO, PLO binds to choles-
terol in membranes, oligomerizes, and forms pores
in the membrane (Sekino-Suzuki et al. 1996). As a
virulence factor specific to this organism (Feil et al.
1996), the toxin is a suitable target for use in detect-
ing the presence of this foodborne pathogen.STAPHYLOCOCCUS AUREUSStaphylococcus aureusis a Gram-positive microor-
ganism found ubiquitously in the environment.
More than 30% of healthy humans carry S. aureus
(Menestrina et al. 2001). The most commonly con-
taminated foods are poultry and meat or products
produced from them, milk, and seafood (Balaban
and Rasooly 2000). Most outbreaks of illness from
this organism occur during the late summer when