Final answer:
Listeria monocytogenes is a pathogenic bacterium causing listeriosis, which survives in refrigerated and salty conditions, facilitating food contamination. It has specialized surface proteins that enable cell envelope modifications, actin polymerization for motility, and evasion of the immune system. Listeriosis is diagnosed by cultivating the pathogen from sterile body sites and identifying it using various laboratory techniques.
Step-by-step explanation:
Listeria monocytogenes is a gram-positive bacterium that causes listeriosis, a serious infection primarily transmitted through consumption of contaminated food. L. monocytogenes is known for its ability to survive in harsh conditions such as refrigeration and high-salt environments, which makes it a frequent source of food poisoning. The pathogen expresses specialized surface proteins that enable it to modify its cell envelope and aid in its pathogenicity. These proteins include internalins, such as InlA and InlB, which facilitate the crossing of intestinal and other critical barriers within the host. To propagate within the host's cells, L. monocytogenes utilizes a protein called ActA to induce actin polymerization. This allows the bacterium to form actin tails for motility and cell-to-cell spread, effectively avoiding the extracellular immune defenses.
Other virulence factors, such as listeriolysin O (LLO), produced by the hyl gene, contribute to the ability of L. monocytogenes to escape from vacuoles after phagocytosis. The pathogen can cross the intestinal barrier and disseminate through the body, potentially leading to meningitis as it penetrates the blood-brain barrier. Identification of L. monocytogenes typically involves cultivation from sterile sites like blood or cerebrospinal fluid (CSF), using techniques such as cold enrichment and pulsed-field gel electrophoresis for typing and subtyping strains.