General Characteristics

The genus Listeria comprises seven species, of which only L. monocytogenes and Listeria ivanovii are considered pathogenic. L. monocytogenes is an important human pathogen, whereas L. ivanovii is primarily an animal pathogen. L. monocytogenes is widespread in the environment and has been recovered from soil, water, vegetation, and animal products. It also has been isolated from crustaceans, flies, and ticks. It has long been known to cause illness in many species of wild and domestic animals, including sheep, cattle,

swine, horses, dogs, cats, rodents, birds, and fish. The first human infection was described relatively recently (1926). Since then, the incidence has increased somewhat, and listeriosis is now recognized as an uncommon but serious infection primarily of neonates, pregnant women, and immunocompromised hosts.

Infection may also occur in healthy individuals.


Virulence Factors

Listeria monocytogenes produces a number of products that have been proposed as virulence factors.

These include hemolysin Oisteriolysin 0), catalase, superoxide dismutase, phospholipase C, and a surface protein, p60. Protein p60 induces phagocytosis through increased adhesion and penetration into mammalian cells. Listeriolysin 0 damages the phagosome membrane, effectively preventing killing of the organism by the macrophage. The correlation between listeriolysin o production and virulence is strong. Nonhemolytic isolates are found to be avirulent and demonstrate no intracellular spread of the organism.

Clinical Infections

The infectious dose and portal of entry of listeriosis have not been determined, but animal studies as well as analysis of human outbreaks seem to indicate that

the ingestion of contaminated food with subsequent systemic spread through the intestine is likely. The clinical manifestations of listeriosis differ among patient

groups. Infections of newborns and immunocompromised adults are the most common, but disease in healthy individuals, particularly in pregnant women,

also occurs.

Disease in Pregnant Women

During pregnancy, listeriosis is most commonly seen during the third trimester. It has been postulated that L. monocytogenes is responsible for spontaneous abortion and stillborn neonates. A pregnant woman with listeriosis may experience a flulike illness, with fever, headache, and myalgia. At this point, the organism is in the bloodstream and has seeded the uterus and fetus. It may progress and result in premature labor or septic abortion within 3 to 7 days. It appears that the infection often is self-limited, because the source of the infection is eliminated when birth occurs.

Disease in the Newborn

Infection of the neonate with L. monocytogenes .is extremely serious. Fatality rates are high, approaching 50% if the fetus is born alive. There are two forms of neonatal listeriosis: early onset and late onset. Early onset listeriosis results from an intrauterine infection that can cause illness at or shortly after birth. The result is most often sepsis. Early onset disease may be associated with aspiration of infected amniotic fluid.

Late-onset disease occurs several days to weeks after birth. Affected infants generally are full term and healthy at birth. The disease is most likely to manifest as meningitis. The fatality rate is lower than in early onset infection, although it also is a very serious, often fatal infection.

Disease in the Immunosuppressed Host

Invasive listeriosis most commonly occurs in persons who are immunosuppressed or in older adults, and particularly in patients receiving chemotherapy, as demonstrated in the Case in Point. Young children are also at risk for infection. In older adults and immunocompromised persons, the fatally rate is high.

The most common manifestations are central nervous system infection and endocarditis. Diagnosis is made by culturing L. monocytogenes from the blood or cerebrospinal fluid (CSF).

Infection of apparently healthy individuals may occur through the intestinal tract when they eat food contaminated with L. monocytogenes. Outbreaks have occurred as a result of eating contaminated cheese, coleslaw, and chicken. Recently, contaminated ice cream, hot dogs, and luncheon meats have served as vehicles for this food-borne disease. The antimicrobial agents that have been effectively used to treat listeriosis are the penicillins, aminoglycosides, and macrolides. Resistance is not common, although some strains are resistant to one or more agent.

Laboratory Diagnosis Microscopy

In direct smears (Figure 16-5), L. monocytogenes appears as a gram-positive coccobacillus. With subculturing,

it tends to appear as coccoid forms. Older cultures often appear gram-variable. Cells may be found singly, in short chains, or in palisades. Depending upon

the cultural conditions, L. monocytogenes may resemble Streptococcus when found in the coccoid form and Corynebacterium when the bacillus forms prevail.

Organisms are not usually seen on the CSFsmear.

Cultural Characteristics

L. monocytogenes grows well on SBA and chocolate agar, as well as on nutrient agars and in broths, such as brain-heart infusion and thioglycolate. The organism prefers a slightly increased CO2tension for isolation.

The colonies are small, round, smooth, and translucent.

They are surrounded by a narrow zone of  I3-hemolysis,

which may be visualized only if the colony is removed. The colonies and hemolysis are similar to those seen with Streptococcus agalactiae or group B streptococci (Figure 16-6, A). Growth is normally complete in 1 or 2 days.

The optimal growth temperature for L. monocytogenes is 30° to 35° C, but growth occurs over a wide range (0.5° to 45° C). Because L. monocytogenes grows at 4° C, an unusual characteristic, a technique called cold enrichment may be used to isolate the organism from clinical specimens. This technique calls for inoculation of the specimen into broth and incubation at 4° C for several weeks. Subcultures are made at weekly intervals and examined for L. monocytogenes.

The length of time required for isolation using this technique lessens its importance in the clinical setting, because treatment must begin early in the infectious process.


The diagnosis of listeriosis depends upon isolating L. monocytogenes from blood, CSF, or swabs of lesions.

Table 16-2 lists the characteristics of L. monocytogenes and bacteria with similar colony morphologies. L. monocytogenes is catalase positive and motile at room

temperature, which, along with B-hemolysis, excludes the corynebacteria. In wet mount preparations, L. monocytogenes exhibits tumbling motility (end-overend) when viewed microscopically. In motility medium, the characteristic “umbrella” pattern is seen when the organism is incubated at room temperature (22° to 25° C) but not at 35° C (Figure 16-7).

L. monocytogenes is hippurate hydrolysis and bile esculin hydrolysis positive. L. monocytogenes also gives a positive CAMP reaction when Staphylococcus

aureus is used to augment the enhanced hemolysis. A more pronounced CAMP reaction is seen with L. monocytogenes when R. equi is used in place of S. aureus. L. monocytogenes produces a “block” type hemolysis with the CAMP test. This type of hemolysis is in contrast to the “arrowhead” type produced by

group B streptococci (see Figure 16-6, B). The positive CAMP reaction distinguishes L. monocytogenes from the other Listeria spp., which are CAMP negative.

L. monocytogenes is differentiated from group B streptococci and the enterococci by a positive catalase test and motility. A presumptive identification can be made based on the results of a Gram stain, tumbling motility, positive catalase, and esculin hydrolysis. Confirmatory tests are acid production from glucose and positive Voges-Proskauer and methyl red reactions.

figure 16-5 figure 16-6 FIGURE 16-7 table 16-2

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