Listeria monocytogenes is a bacterium of significant public health concern, primarily recognized as a foodborne pathogen. This microorganism possesses unique biological attributes that enable its survival across diverse environments and its ability to cause severe illness, known as listeriosis. Understanding these properties is crucial for controlling its spread and mitigating the risks it poses to human health.
Defining Features of Listeria monocytogenes
Listeria monocytogenes is classified as a Gram-positive bacterium, which means it retains a crystal violet stain due to its thick peptidoglycan cell wall. It is a short, rod-shaped bacterium, often appearing in single cells or short chains. A distinguishing metabolic characteristic is its facultative anaerobic nature, meaning it can grow and survive both in the presence and absence of oxygen.
This bacterium is also notably psychrotrophic, meaning it can grow at low temperatures, including those found in refrigeration, with an optimal growth temperature between 30-37°C. This cold tolerance allows it to multiply slowly in refrigerated foods, presenting a challenge for food safety. Listeria monocytogenes exhibits a distinctive “tumbling motility” at temperatures between 20-25°C. However, at human body temperature (37°C), it shows reduced or no motility.
How Listeria Survives and Spreads
Listeria monocytogenes is ubiquitous in nature, commonly found in various environmental reservoirs such as soil, water, and decaying vegetation. Its presence extends to the feces of healthy animals and humans, contributing to its widespread distribution. This bacterium can colonize farm animals, which then shed it into the agricultural environment, creating a pathway for its introduction into food processing environments.
The resilience of Listeria monocytogenes allows it to persist in food processing facilities for extended periods. This persistence is often linked to its ability to form biofilms, which are communities of bacteria encased in a self-produced protective matrix that adhere firmly to surfaces. Biofilms can develop on various food contact surfaces, making the bacteria within them more resistant to cleaning and disinfection efforts. This characteristic poses a challenge for controlling contamination in the food industry, especially for ready-to-eat foods like deli meats, soft cheeses, and smoked fish.
The Mechanisms of Listeria Infection
Listeria monocytogenes employs specific virulence factors to invade and multiply within host cells, enabling it to cause disease. Internalins mediate the bacterium’s binding to host cells and induce its uptake, even by non-phagocytic cells. Once inside a host cell, the bacterium is enclosed within a vacuole.
To escape this vacuole, Listeria monocytogenes produces listeriolysin O (LLO), a pore-forming toxin. The acidic environment within the vacuole activates LLO, allowing it to lyse the vacuolar membrane and release the bacterium into the host cell’s cytoplasm. Once in the cytoplasm, Listeria monocytogenes utilizes another surface protein, ActA, to induce the polymerization of host cell actin. This actin polymerization forms a “comet tail” that propels the bacterium through the cytoplasm and into adjacent cells without re-entering the extracellular space. This cell-to-cell spread allows Listeria monocytogenes to evade the host’s immune system, as it remains largely hidden from antibodies and other immune components.