Morphological and Cultural Properties
The causative agent of human listeriosis is 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 can produce a capsule. In smears from pure cultures, they are often arranged at angles to each other, resembling the Latin letter 'V' or hieroglyphs.
Regarding respiration, Listeria species are microaerophiles, preferring a carbon dioxide concentration of 5–10%. The temperature optimum for their growth is 37 °C. They are not fastidious and grow on basic nutrient media, though blood enrichment enhances growth, forming a narrow zone of hemolysis on blood agar. They are biochemically active: they ferment glucose with acid production, are catalase-positive, but do not produce indole or hydrogen sulfide.
Virulence Factors and Pathogenesis
The key virulence mechanism of Listeria is its ability to cause incomplete phagocytosis. The bacteria invade both professional macrophages and non-phagocytic endothelial cells. The intracellular life cycle of the pathogen involves the following stages and factors:
- Internalin: A surface cell-wall protein responsible for adhesion and active uptake of the bacterium by the host cell.
- Listeriolysin O: A metalloprotease enzyme that disrupts the phagosomal membrane. This allows the bacterium to escape degradation and enter the macrophage cytosol.
- Once inside the cytosol, the bacterium actively replicates.
- Phospholipases C and hemolysins: Disrupt host cell and erythrocyte membranes, promoting infection dissemination.
Disseminating via lymphatic and hematogenous routes, the pathogen settles in internal organs (CNS, liver, spleen, lymph nodes). In affected tissues, it forms a specific morphological substrate known as a listerioma. This is a necrotic nodular granuloma consisting of mononuclear phagocytes, affected organ cells, and the bacteria themselves.
Epidemiology and Environmental Resistance
Listeriosis is a classic zoonosis. Natural reservoirs include wild and domestic animals, birds, as well as soil and water, where the bacteria can exist as saprophytes or protozoan symbionts. The pathogen can even infect plants through their root system.
The primary route of human transmission is alimentary. Infection is transmitted via raw milk, cheeses, insufficiently cooked meat, and vegetables. This danger is amplified by the pathogen's remarkable temperature resilience: Listeria tolerates freezing well and exhibits psychrophilicity—remaining viable in food products at household refrigerator temperatures (4 °C).
In addition to foodborne transmission, contact transmission (when handling animals), airborne dust transmission, and the highly dangerous vertical (mother-to-fetus) transmission route are possible. Horizontal transmission from person to person is not described. Meanwhile, Listeria is highly sensitive to heat: it is rapidly killed by boiling and standard disinfectants.
Clinical Features and Immunity
The incubation period varies widely—from 3 to 45 days. In immunocompetent individuals, the infection is often mild (presenting as malaise, respiratory syndrome, or pharyngitis). However, in risk groups (pregnant individuals, neonates, immunocompromised persons), severe generalized forms develop: septicemia and meningoencephalitis.
Listeriosis in pregnancy and neonates occupies a special place:
- During transplacental transmission, granulomas form in the placenta, and the pathogen reaches the fetus. The formation of listeriomas in the internal organs of the fetus leads to miscarriage, fetal demise, or severe developmental anomalies.
- During intrapartum transmission (at birth), the process primarily targets the neonatal CNS, causing meningitis within the first three weeks of life with a mortality rate of up to 90%.
Host defense against Listeria is mediated primarily by cell-mediated immunity. Humoral antibodies are produced but lack strong protective function. The immune response is accompanied by delayed-type hypersensitivity (DTH).