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Listeria monocytogenes (page 1)
(This chapter has 3 pages)
© Kenneth Todar, PhD
Listeria monocytogenes is a Gram-positive rod-shaped
It is the agent of listeriosis, a serious infection caused by
food contaminated with the bacteria. Listeriosis has been
recognized as an important public health problem in the United States.
The disease affects primarily pregnant women, newborns, and adults with
weakened immune systems.
Listeriosis is a serious disease for humans; the overt form
the disease has a mortality greater than 25 percent. The two main
manifestations are sepsis and meningitis. Meningitis is often
by encephalitis, a pathology that is unusual for bacterial infections.
Microscopically, Listeria species appear as small,
rods, which are sometimes arranged in short chains. In direct smears
may be coccoid, so they can be mistaken for streptococci. Longer cells
may resemble corynebacteria. Flagella are produced at room
not at 37°C. Hemolytic activity on blood agar has been used as a
to distinguish Listeria monocytogenes among other Listeria
but it is not an absolutely definitive criterion. Further biochemical
may be necessary to distinguish between the different Listeria
catalase-positive rods, the genus Listeria was classified in
family Corynebacteriaceae through the seventh edition of
Bergey's Manual. 16S rRNA cataloging studies of Stackebrandt et
that Listeria monocytogenes was a distinct taxon within the Lactobacillus-Bacillus
branch of the bacterial phylogeny constructed by Woese (1981). In 2001,
the Famiiy Listeriaceae was
created within the expanding Order Bacillales,
which also includes Staphylococcaceae, Bacillaceae and others. Within
phylogeny there are six species of Listeria. The only other
genus in the family is Brochothrix.
Listeria monocytogenes Gram
Natural Habitats of Listeria and Incidence of Disease
Until about 1960, Listeria monocytogenes was thought to be
almost exclusively with infections in animals, and less frequently in
However, in subsequent years, listeriae, including the
species L. monocytogenes and L. ivanovii, began to be
from a variety of sources, and they are now recognized to be widely
in Nature. In addition to humans, at least 42 species of wild and
mammals and 17 avian species, including domestic and game fowl,
harbor listeriae. Listeria monocytogenes is reportedly carried
the intestinal tract of 5-10% of the human population without any
symptoms of disease. Listeriae have also been isolated from
fish, oysters, ticks, and flies.
The term listeriosis encompasses a wide variety of disease
that are similar in animals and humans. Listeria monocytogenes
causes listeriosis in animals and humans; L. ivanovii causes
disease in animals only, mainly sheep. Encephalitis is the most common
form of the disease in ruminant animals. In young animals, visceral or
septicemic infections often occur. Intra-uterine infection of the fetus
via the placenta frequently results in abortion in sheep and cattle.
The true incidence of listeriosis in humans is not known, because in
the average healthy adult, infections are usually asymptomatic, or at
produce a mild influenza-like disease. Clinical features range from
influenza-like symptoms to meningitis and/or meningoencephalitis.
is most likely to occur in pregnant women, neonates, the elderly and
individuals, but apparently healthy individuals may also be affected.
the serious (overt) form of the disease, meningitis, frequently
by septicemia, is the most commonly encountered disease manifestation.
In pregnant women, however, even though the most usual symptom is a
influenza-like illness without meningitis, infection of the fetus is
common and can lead to abortion, stillbirth, or delivery of an acutely
In humans, overt listeriosis following infection with L.
is usually sporadic, but outbreaks of epidemic proportions have
In 1981, there was an outbreak that involved over 100 people in Canada.
Thirty-four of the infections occurred in pregnant women, among whom
were nine stillbirths, 23 infants born infected, and two live healthy
Among 77 non pregnant adults who developed overt disease, there was
30% mortality. The source of the outbreak was coleslaw produced by a
One of the most serious and publicized outbreaks of listeriosis occurred in California in 1985 as reported in MMWR, June 21, 1985 / 34(24);357-9.
In 2002, a multistate outbreak of Listeria
with 46 culture-confirmed cases, seven deaths, and three stillbirths or
miscarriages in eight states was linked to eating sliced turkey
deli meat. One intact food product and 25 environmental samples from a
poultry processing plant yielded L.
monocytogenes. Two environmental
isolates from floor drains were indistinguishable from that of outbreak
patient isolates, suggesting that the plant might be the source of the
According to the report,
between January 1 and June 14, 1985, 86 cases of Listeria monocytogenes infection were identified in Los Angeles and Orange Counties, California. Fifty-eight of the cases were among mother-infant pairs. Twenty-nine deaths occurred: eight neonatal deaths, 13 stillbirths, and eight non-neonatal deaths. The increased occurrence of listeriosis was first noted at the Los Angeles County-University of Southern California Medical Center; all cases were in pregnant Hispanics, and all appeared to be community-acquired. A systematic review of laboratory records at hospitals in Los Angeles and Orange County identified additional cases throughout the area.
An analysis of Los Angeles County cases showed that 45 (63%) of the Listeria cases were among mother-newborn pairs. Most (70%) of these women had a prior febrile illness or were febrile on admission to the hospital. Forty-two of the neonatal patients had onset of disease within 24 hours of birth, and all isolates available for testing were serotype 4b. Three of the neonatal patients had late onset disease; only one of the two isolates available for testing was serotype 4b.
Samples of Mexican-style cheeses from three different manufacturers purchased from markets in Los Angeles were cultured at CDC; four packages of one brand of cheese grew L. monocytogenes serotype 4b. The four positive cheese samples were of two varieties, queso fresco and cotija.