Listeria

ORGANISM:

GENERAL CONCEPTS:

Listeria monocytogenes is the agent responsible for listeriosis, a food-borne disease that can be very serious in humans.
Disease is generally restricted to the elderly, pregnant women, infants and immunocompromised individuals.
The major clinical manifestations of listeriosis are sepsis and meningitis.
Pregnant women who become infected may experience a mild flu-like illness but the fetus may be infected transplacentally, causing miscarriage or stillbirth.

DISTINCTIVE PROPERTIES:

Listeria are small, Gram-positive bacilli or coccobacilli sometimes seen in chains (cf. Streptococcus) or diphtheriod-like (cf. Corynebacterium).
The organisms are generally beta-hemolytic, catalase-positive, and grow well at 4°C.
They are motile at room temperature, but do not produce flagella at 37°C.
Listeria are intracellular pathogens whose virulence factors (e.g. hemolysin, phospholipase C) are thermoregulated.

PATHOGENESIS:

Life-cycle of Listeria monocytogenes
Cartoon cropped from Tilney and Portnoy (2002)

Listeria monocytogenes is ingested with contaminated food.
Generally, the organisms are destroyed by host defenses but they are able to survive within macrophages via escape from the phagolysosome, allowing intracellular replication. They are, therefore, facultative intracellular parasites.
The bacteria are capable of polymerizing the host cell's actin to produce motility and this allows spread from cell to cell without an extracellular stage.
Growth within the macrophages may carry the organisms to the brain or transplacentally, resulting in meningitis or fetal infection, respectively.
The mortality of listeriosis may exceed 20%.

HOST DEFENSES:

Since the organisms are largely intracellular, cell mediated defenses (activated macrophages, CD8⁺ T-cells) are most important.

EPIDEMIOLOGY:

The organisms may be found in a variety of places including animals and soil.
Contamination leading to disease outbreaks has been associated with hot dogs, various meats, milk, cheese and raw vegetables.
A recent outbreak (2011) was traced to cantaloupes from Colorado.

DIAGNOSIS:

Clinical: Not possible
Laboratory: The organisms may readily be cultured from blood or CSF. Small, catalase-positive, beta-hemolytic colonies appear after 48 hours. "Cold-enrichment" (placing the samples at 4°C for several months) may help recovery from difficult specimens.

CONTROL: