Final answer:
L. monocytogenes is a gram-positive bacterium that is a frequent source of food poisoning. It exhibits virulence by invading cells, using virulence factors to spread within the host, and breaching barriers such as the blood-brain barrier and the placenta. L. monocytogenes is able to tolerate refrigeration temperatures and high concentrations of salt, making it a persistent contaminant in food.
Step-by-step explanation:
L. monocytogenes is a gram-positive short rod found in soil, water, and food. It is classified as a psychrophile and is halotolerant. Its ability to multiply at refrigeration temperatures (4-10 °C) and its tolerance for high concentrations of salt (up to 10% sodium chloride [NaCl]) make it a frequent source of food poisoning.
Once ingested, L. monocytogenes invades intestinal epithelial cells, translocates to the liver, and grows inside hepatic cells. It uses virulence factors such as listeriolysin O (LLO) and actin assembly-inducing protein (ActA) to escape vacuoles, polymerize host actin, produce actin tails, move around the cell's cytoplasm, and spread from cell to cell without exiting into the extracellular compartment.
In addition, L. monocytogenes produces surface proteins called internalins (InlA and InlB) that help it invade nonphagocytic cells and tissues, allowing it to penetrate the intestinal wall and disseminate through the circulatory and lymphatic systems. Internalins also enable L. monocytogenes to breach important barriers, including the blood-brain barrier and the placenta.