This microorganism is a facultative gram-
positive, rod-shaped, non-spore-forming
microaerophilic (5 to 10% CO 2 ) bacterium.
L. monocytogenes,a ubiquitous pathogen,
occurs in human carriers (ca. 10% of the
population) and is found in the intestinal
tracts of over 50 domestic and wild species
of birds and animals, including sheep, cattle,
chickens, and swine, as well as in soil and
decaying vegetation. Other potential sources
of this microorganism are stream water,
sewage, mud, trout, crustaceans, houseflies,
ticks, and the intestinal tracts of sympto-
matic human carriers. This pathogen has
been found in most foods, from chocolate
and garlic bread to diary products and meat
and poultry. Elimination of Listeria is
impractical and may be impossible. The crit-
ical issue is how to control its survival.
The optimal temperature range for the
proliferation of this microbe is 30 to 37ºC;
however, growth can occur at a temperature
range of 0 to 45ºC. This microorganism is
considered to be a psychrotrophic pathogen,
which grows well in damp environments.L.
monocytogenesis very tolerant of environ-
mental stresses compared to other vegetative
cells and has a high vegetative cell heat resist-
ance. It grows in over 10% salt and survives
in saturated salt solutions. This pathogen
will grow twice as fast at 10ºC as at 4ºC, sur-
vive freezing temperatures, and is usually
destroyed at processing temperatures above
61.5ºC. Although L. monocytogenesis most
frequently found in milk, cheese, and other
dairy products, it can be present in vegeta-
bles that have been fertilized with the manure
of infected animals. This microorganism
thrives in substrates of neutral to alkaline
pH but not in highly acidic environments.
Growth can occur in a pH range from 5.0 to
9.6, depending on the substrate and temper-
ature.L. monocytogenesoperates through
intracellular growth in mononuclear phago-
cytes. Once the bacterium enters the host’s,
monocytes, macrophages, or polymorphonu-
cleus leukocytes, it can evade host defenses
and grow.
Human listeriosis may be caused by any of
13 serotypes ofL. monocytogenes, but those
most likely to cause illness are 1/2a, 1/2b, and
4b (Farber and Peterkin, 2000). Most cases
of listeriosis are sporadic. This illness prima-
rily affects pregnant women, infants, people
over 50 years old, those debilitated by a dis-
ease, and other individuals who are in an
immunocompromised state of health.
Meningitis or meningoencephalitis is the
most common manifestations of this disease
in adults. This disease may occur as a mild
illness with influenza-like symptoms, sep-
ticemia, endocarditis, abscesses, osteomyelitis,
encephalitis, local lesions, or minigranulomas
(in the spleen, gall bladder, skin, and lymph
nodes) and fever. Fetuses of pregnant women
with this disease may also be infected. These
women might suffer an interrupted pregnancy
or give birth to a stillborn child. Infants who
survive birth may be born with septicemia or
develop meningitis in the neonatal period.
The fatality rate is approximately 30% in new-
born infants and nearly 50% when the infec-
tion occurs in the first 4 days after birth.
Mascola et al. (1988) reported that liste-
riosis is dangerous to persons with AIDS.
Because AIDS severely damages the immune
system, those with the disease are more sus-
ceptible to a foodborne illness such as liste-
riosis (Archer, 1988). Mascola et al. (1988)
reported that AIDS-diagnosed males are
more than 300 times as susceptible to liste-
riosis as those of the same age who were
AIDS-negative. The infectious dose for
L. monocytogeneshas not been established
because of the presence of unknown factors
in persons with normal immune systems that
make them less susceptible to the bacteria
than immunocompromised persons. The
infectious dose depends on both the strains
ofListeriaand on the individual. However,The Relationship of Microorganisms to Sanitation 45