LISTERIA MONOCYTOGENES: CHARACTERISTICS, MORPHOLOGY AND PATHOGENESIS - BIOLOGY - 2025

Listeria monocytogenes is a pathogenic bacterium, commonly associated with food contamination. It has a wide distribution worldwide in environments such as soils, fresh and sewage waters, vegetation and fecal matter. It can infect humans, cattle, goats, sheep, birds (turkeys, chickens, pheasants, web-footed), fish and crustaceans.

The transmission of this bacterium in humans occurs mainly through the ingestion of contaminated foods of animal and vegetable origin, fresh and processed, unpasteurized milk and dairy products, pork, beef, poultry and fish. Mainly those foods that are consumed fresh or with long periods of refrigeration.

Electron micrograph of L. monocytogenes, 41,250X. By Elizabeth White, via Wikimedia Commons

It can also be transmitted by direct contact with infected animals, their tissues, their feces or their contaminated environment (zoonosis), by horizontal contamination (mother-child) or by intra-hospital or nosocomial contamination in obstetric and gynecological activities.

Listeriosis is a rare disease (they occur in 0.1 to 10 cases per year per million people) that can become serious in pregnant women, the elderly, infants and immunosuppressed people, such as patients with HIV / AIDS, leukemia, cancer, transplants kidney or corticosteroid therapy.

By presenting a mortality rate of 20 to 30%, it is considered by the World Health Organization as an important public health problem.

General characteristics

-L. Monocytogenes bacteria are gram-positive, motile, non-sporulated, facultative anaerobic and pathogenic cocobacilli.

-It has a facultative anaerobic metabolism.

-They are catalase positive and oxidase negative.

-They are capable of surviving in a wide range of temperatures (from -18 to 50 ºC) and pH (from 3.3 to 9.6) and tolerate salt concentrations of 20%.

-They are distributed worldwide, in a great diversity of environments. This wide distribution is due to its ability to survive for long periods of time in different media, under very broad conditions of temperature, pH and salinity.

-These same characteristics give it great potential to contaminate food in any link of the production chain, including during cold storage.

Taxonomy

L. monocytogenes is a bacterium belonging to the phylum Firmicutes and to the order of Bacillales. It was described in 1926 as Bacterium monocytogenes, renamed Listerella hepatolitica in 1927 and finally named Listeria monocytogenes in 1940.

It was the only species recognized for the genus until 1961. Currently 17 species of Listerella are recognized, 9 of which have been described after 2009.

Its specific epithet is due to the ability of its membrane extracts to stimulate the production of monocytes in laboratory infected rabbits and guinea pigs.

Morphology

L. monocytogenes is rod-shaped and can be 0.4 to 0.5 microns wide by 0.5 to 1.2 microns long.

It has peritrichous flagella, which give it a particular mobility, which are inactivated above 37ºC.

Pathogeny

The pathogenicity of L. monocytogenes is a result of its ability to adhere, invade, and multiply within different non-phagocytic cells.

Colonization of host tissues begins, in most cases, after ingestion of contaminated food. In the stomach, L. monocytogenes must support proteolytic enzymes, gastric acid and bile salts, for which it induces at least 13 oxidative stress proteins and 14 toxic “shock” proteins.

Later, the L. monocytogenes cells overcome the intestinal barrier through the blood and lymph, reaching the lymph nodes, the spleen and the liver. Bacteria multiply mainly in hepatocytes. The passage from hepatocyte to hepatocyte produces an infectious focus in which the bacteria spread through the liver parenchyma.

L. monocytogenes is capable of infecting a wide variety of tissues in the host. However, there is evidence that this organism prefers the gravid uterus and the central nervous system.

In humans, infection of the placenta occurs by colonization of the trophoblastic membrane and subsequent translocation of the endothelial barrier. Through this route, the bacteria reach the fetal bloodstream, producing a generalized infection that leads to the death of the fetus in utero or the premature death of the infected neonate.

Finally, infection of the central nervous system occurs by centripetal migration along the cranial nerves, producing meningitis, associated with the presence of infectious foci in the brain parenchyma, especially in the brain stem, with macroscopic lesions restricted to the soft brain and to the cerebellum.

Listeriosis

Infection with L. monocytogenes is called listeriosis. It usually occurs as an asymptomatic infection and has a relatively low occurrence.

Noninvasive listeriosis causes acute febrile gastroenteritis. It is the mild form that mainly affects healthy people. It is associated with the ingestion of foods contaminated with high populations of L. monocytogenes. It has a short incubation period. Non-invasive listeriosis cases generate the following symptoms:

• Fever.
• Headaches
• Sickness.
• Vomiting
• Abdominal pain.
• Diarrhea.
• Myalgia.

Invasive listeriosis is associated with high-risk groups, such as pregnant women, the elderly, infants, and immunosuppressed people, such as patients with HIV / AIDS, leukemia, cancer, kidney transplants, or corticosteroid therapy.

Characteristics of listeriosis

It is characterized by a high mortality rate (20-30%). The incubation period lasts between 1 and 2 weeks, but can last up to 3 months.

It can cause skin eruptions in the form of papules or pustules on the arms or hands, frequently associated with contact with infected animals; conjunctivitis and inflammation of the lymph nodes in front of the ears and in the most complicated cases can cause meningitis, meningoencephalitis and sometimes rhomboencephalitis.

Other forms of the disease can cause arthritis, endocarditis, endophthalmitis, peritonitis, pleural effusions, internal and external abscesses, among others.

Likewise, it can cause spontaneous abortion or fetal death, in cases of infections in the uterus and fetus of pregnant women. In newborns it can also cause low birth weight, septicemia, meningitis or meningoencephalitis.

Cases of invasive listeriosis can generate several of the symptoms mentioned below:

• Fever.
• Headaches
• Sickness.
• Vomiting
• Abdominal pain.
• Diarrhea.
• Myalgia.

Treatment

The most commonly used treatment for L. monocytogenes infections is a combination of gentamicin with broad-spectrum penicillins such as ampicillin.

The combination of trimethoprim and sulfamethoxazole has also been used in patients allergic to penicillin. Aminoglycosides are also often used in patients with meningoencephalitis, together with the base treatment of penicillin or ampicillin.

However, the effectiveness of the treatment depends on the strains, since it is a bacterium capable of creating resistance to antibiotics and multi-resistance.

A recent study reveals that of 259 L. monocytogenes strains, 145 were resistant to multiple drugs, being mainly resistant to daptomycin, tigecycline, tetracycline, ciprofloxacin, ceftriaxone, trimethoprim / sulfamethoxazole, and gentamicin.

References

1. DataBio. 2017. Listeria monocytogenes. National Institute of Safety and Hygiene at Work.
2. Farber, JM & Peterkin, PI 1991. Listeria monocytogenes, a food-borne pathogen. Microbiological Reviews 55 (3): 476-511.
3. Basque Foundation for Food Safety. 2006. Listeria monocytogenes. Madrid.
4. Listeria monocytogenes. (2018, July 19). Wikipedia, The Free Encyclopedia. Consultation date: 20:20, September 27, 2018 from es.wikipedia.org.
5. Nollab, M., Kletab, S. & Al Dahoukbc, S. (2018). Antibiotic susceptibility of 259 Listeria monocytogenes strains isolated from food, food-processing plants and human samples in Germany. Journal of Infection and Public Health, 11 (4): 572-577.
6. World Health Organization. (2017). Listeriosis Date consulted: September 27, 2018 from who.int.
7. Orsi, RH & Wiedmann, M. 2016. Characteristics and distribution of Listeria spp., Including Listeria species newly described since 2009. Applied Microbiology and Biotechnology 100: 5273–5287.
8. Torres, K., Sierra, S., Poutou, R., Carrascal, A. & Mercado, M. 2005. Pathogenesis of Listeria monocytogenes, emerging zoonotic microorganism. Magazine MVZ Córdoba 10 (1): 511-543.