Listeria

Morphological and cultural properties

The causative agent of human listeriosis is the species Listeria monocytogenes, discovered in 1911 and named after the surgeon Joseph Lister. These are small, Gram-positive, motile rods that do not form spores but are capable of forming a capsule. In smears from pure culture, they are often arranged at an angle to each other, resembling the Latin letter 'V' or hieroglyphs.

In terms of respiration, listerias are microaerophiles that prefer a carbon dioxide concentration of 5–10%. The optimum temperature for their growth is 37 °C. They are undemanding and grow on simple nutrient media, although the addition of blood improves growth, with colonies forming a narrow zone of hemolysis on blood agar. They are biochemically active: they ferment glucose with acid production, produce catalase (catalase-positive), but do not produce indole or hydrogen sulfide.

Pathogenicity factors and pathogenesis

The key virulence mechanism of listerias is their ability to cause incomplete phagocytosis. The bacteria penetrate both professional macrophages and non-phagocytic endothelial cells. The intracellular life cycle of the pathogen includes the following stages and factors:

• Internalin: a cell wall surface protein responsible for adhesion and active uptake of the bacterium by the host cell.
• Listeriolysin: a metalloprotease enzyme that disrupts the phagosomal membrane. This allows the bacterium to evade digestion and escape into the macrophage cytosol.
• Inside the cytosol, the bacterium actively replicates.
• Phospholipases C and hemolysins: disrupt cell membranes of cells and erythrocytes, promoting the dissemination of the infection.

Spreading via lymphatic and hematogenous routes, the pathogen settles in internal organs (CNS, liver, spleen, lymph nodes). A specific morphological substrate forms in the affected tissues — a listerioma. This is a necrotic granuloma nodule consisting of mononuclear phagocytes, cells of the affected organ, and the bacteria themselves.

Epidemiology and environmental persistence

Listeriosis is a typical zoonosis. The natural reservoir consists of wild and domestic animals, birds, as well as soil and water, where bacteria can exist as saprophytes or protozoan symbionts. The pathogen is even capable of infecting plants via the root system.

The main route of human infection is alimentary. The infection is transmitted through raw milk, cheeses, insufficiently heat-treated meat, and vegetables. The danger is compounded by the pathogen's incredible temperature resistance: listerias tolerate freezing well and exhibit psychrophilicity, retaining viability in food products at household refrigerator temperatures (4 °C).

In addition to the foodborne route, contact transmission (when caring for animals), airborne dust transmission, and the highly dangerous vertical (mother-to-fetus) transmission route are possible. Horizontal human-to-human transmission has not been described. At the same time, listerias are very sensitive to heat: they are rapidly killed by boiling and treatment with disinfectants.

Infection characteristics and immunity

The incubation period of the disease varies widely, from 3 to 45 days. In individuals with normal immunity, the infection often runs a mild course (manifesting as malaise, respiratory syndrome, or tonsillitis). However, in risk groups (pregnant women, newborns, immunocompromised individuals), severe generalized forms develop: septicemia and encephalomeningitis.

A special place is occupied by listeriosis in pregnant women and newborns:

1. During transplacental transmission, granulomas form in the placenta, and the pathogen penetrates to the fetus. The formation of listeriomas in the internal organs of the fetus leads to miscarriages, fetal death, or severe developmental anomalies.
2. During intranatally acquired infection (during childbirth), the process primarily attacks the newborn's central nervous system, causing meningitis in the first three weeks of life with a mortality rate of up to 90%.

Body defense against listerias is provided predominantly by cellular immunity. Humoral antibodies are produced but do not possess a pronounced protective function. The immune response is accompanied by delayed-type hypersensitivity (DTH).

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Molecular biological mechanism of action of listeriolysin and phospholipase C
• Detailed algorithm for bacteriological diagnosis of listeriosis
• Features of granuloma (listerioma) formation in fetal tissues during intrauterine infection
• Antigenic structure of listerias: characteristics of 7 L. monocytogenes serovars
• Specifics of the cellular immune response (DTH) in intracellular infections

Other topics in the section 'Regular Gram-Positive Rods (Lactobacilli, Listerias)'

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