What is Listeria?

Definition

The bacterium Listeria monocytogenes commonly contaminates
foods. It is difficult to detect and, in rare cases, causes listeriosis,
a serious and often fatal disease in humans.

Natural Habitat and Features

Listeria species are found in soil, decaying vegetation, water, sewage, and field crops such as alfalfa. The only pathogenic
species, L. monocytogenes, occurs in humans, other mammals,
birds, fish, crustaceans, and insects. L. monocytogenes also
occurs in foods such as vegetables, poultry, fresh and processed meats (such as
bologna and hot dogs), soft cheeses (such as feta, farmer’s cheese, and
queso blanco), and salad dressings. It occurs in raw foods and
in processed products. The first three confirmed outbreaks of listeriosis, in the
1980’s, were reported from ingesting coleslaw, milk, and Mexican-style cheese
(queso fresco), respectively.

Listeria spp. are characterized as gram-positive, non-spore-forming rods, which may occur in short chains or in coccoid forms. They are facultative anaerobes, are catalase-positive, always ferment, and ferment sugars to produce acid. Their cell walls contain teichoic acids but not mycolic acids. An unusual property of L. monocytogenes is that it can be intracellular in animal cells.

L. monocytogenes grows slowly under refrigerated conditions and
is resistant to cold, acid, and salt. These properties increase the risk of
infection from ingesting processed and stored foods that may
be contaminated.

L. innocua has a significantly larger genome than L. monocytogenes (3.01 megabases compared with 2.94 megabases), though L. innocua lacks some virulence genes and is nonpathogenic. Both genomes code for about 3,000 proteins, with perhaps 100 to 120 more in L. innocua. Thus, L. innocua grows in a wide variety of environments and is easier than L. monocytogenes to detect. Because of many genetic and metabolic similarities, and because they are often found together in a variety of situations, L. innocua could be used as an indicator of L. monocytogenes in quality control and regulatory analyses of foods. The two species grow slowly and indistinguishably on many laboratory media but can be distinguished by PCR (polymerase chain reaction) and colony morphology on special blood agars. Pulse-field gel electrophoresis of genomic deoxyribonucleic acid (DNA) and ribotyping are also used to distinguish Listeria spp.

Of the thirteen serovars of L. monocytogenes, listeriosis is most often caused by strains of just three: serovars 4b, 1/2a, and 1/2b. In the lab, serovars are distinguished by serotyping and phagetyping.

Pathogenicity and Clinical Significance

Even as a food-borne disease, listeriosis is not mainly a gastrointestinal
illness. Instead, severe symptoms include meningitis,
sepsis, and abortion in women in late-term pregnancy. Many people who ingest
L. monocytogenes do not show any symptoms but may carry
bacteria and act as a source of food contamination. The disease is especially
risky for infants, the elderly and bedridden, and persons with acquired immunodeficiency
syndrome (AIDS) or other immune disorders. Incidence of
listeriosis is about five cases per million people per year, probably worldwide.
Case fatality is quite severe at 20 to 30 percent. Thus, the United States reports
about five hundred deaths per year from listeriosis. Most cases are sporadic, and
epidemics are rare.

The virulence of L. monocytogenes is an inherent property of the bacterium because of virulence genes carried in its genome. After entering the gastrointestinal tract, bacteria are attacked by phagocytes but resist destruction. Instead, they persist intracellularly, proliferate inside phagocytes, and release progeny when phagocytes lyse, or break apart. The bacterial pore-forming protein listeriolysin O facilitates cell lysis of phagocytic cells. The bacterial surface protein internalin (InlA) aids in the crossing of the intestinal barrier to enter the nervous system. Other bacterial virulence proteins are InlB, ActA, PlcA, and PlcB. These proteins (with InlA and listeriolysin O) are encoded by virulence genes, some linked in a 10-kilobase region of the L. monocytogenes genome, absent from L. innocua.

Drug Susceptibility

Listeria spp. are susceptible to many different antibiotics,
though resistant strains are emerging. L. monocytogenes infection
in humans can usually be treated with ampicillin, especially if treatment starts
early. No vaccine is available.

Bibliography

Bell, Chris, and Alec Kyriakides. Listeria: A Practical Approach to the Organism and Its Control in Foods. 2d ed. Ames: Iowa State University Press, 2005.

Bibek, Ray, and Arun Bhunia. Fundamental Food Microbiology. 4th ed. Boca Raton, Fla.: CRC Press, 2007.

Doumith, Michel, et al. “New Aspects Regarding Evolution and Virulence of Listeria monocytogenes Revealed by Comparative Genomics and DNA Arrays.” Infection and Immunity 72 (2004): 1072-1083.

Glaser, Philippe, et al. “Comparative Genomics of Listeria Species.” Science 294 (2001): 849-852.

Montville, Thomas J., and Karl R. Matthews. Food Microbiology: An Introduction. 2d ed. Washington, D.C.: ASM Press, 2008.