it appears that thousands or even millions of
cells may be required to infect healthy ani-
mals, whereas 1 to 100 cells may infect those
who are immunocompromised. The severe
form of human listeriosis usually does not
occur in the absence of a predisposing infec-
tion, although it has been reported that
L. monocytogenescan cause gastroenteritis
in previously healthy individuals.
L. monocytogenescan adhere to food con-
tact surfaces by producing attachment fib-
rils, with the subsequent formation of a
biofilm, which impedes removal during
cleaning. The attachment ofListeriato solid
surfaces involves two phases. They are pri-
mary attraction of the cells to the surface
and firm attachment following an incubation
period. A primary acidic polysaccharide is
responsible for initial bacterial adhesion.
This microbe adheres by producing a mass
of tangled polysaccharide fibers that extend
from the bacterial surface to form a “glyco-
calyx,” which surrounds the cell of the
colony and functions to channel nutrients
into the cell and to release enzymes and tox-
ins. These microbes are also potential con-
taminants of raw materials utilized in plants,
which contribute to constant reintroduction
of this organism into the plant environment.
Utilization of Hazard Analysis Critical Con-
trol Points (HACCP) and other process con-
trol practices is the most effective method of
controlling this pathogen in the processing
environment. The HACCP approach has
helped to identify critical points and to eval-
uate the effectiveness of control systems
through verification procedures.
This pathogen is most effectively transmit-
ted through the consumption of contami-
nated food, but it can also occur from
person-to-person contact or by inhalation of
this microorganism. For example, a person
who has had direct contact with infected
materials, such as animals, soil, or feces, may
develop lesions on the hands and arms. This
pathogen is likely to be found in home refrig-
erators, suggesting the need for regular
cleaning and sanitizing of this equipment.
A study reported by the Center for Dis-
ease Control and Prevention (Felix, 1992)
found Listeriaspecies present in 64% of 123
home refrigerators that were checked. The
most effective prevention against listeriosis is
to avoid the consumption of raw milk, raw
meat, and foods made from contaminated
ingredients. It is important for pregnant
women, especially, to avoid contact with
infected animals. Fail-safe procedures for the
production ofListeria-free products have
not been developed. Thus, food processors
must rely on a rigid environmental sanitation
program and HACCP principles to establish
a controlled process. The most critical areas
for the prevention of contamination are
plant design and functional layout, equip-
ment design, process control operational
practices, sanitation practices, and verifica-
tion ofL. monocytogenescontrol.
Various studies have demonstrated that
L. monocytogenesis resistant to the effects of
sanitizers. This pathogen has resistance to
the effects of trisodium phosphate (TSP),
and exposure to a high (8%) level of TSP for
10 minutes at room temperature is required
to reduce bacterial numbers by 1 log after a
colony has grown on the surface and a
biofilm has formed. Furthermore, washing
skin with 0.5% sodium hydroxide has a mini-
mal effect on the proliferation ofL. monocy-
togenes. This microorganism is more
resistant to the cooking process than are
other pathogens, and cooking may not be a
definitive means of eliminating the organism
from foods. Although L. monocytogenesis
susceptible to irradiation, it is not the final
solution with regard to eliminating this
pathogen from fresh meat and poultry.
Russell (1997) has recognized that
although a minimal number of listeriosis
cases are reported in the United States each