**Core Concept**
Listeria monocytogenes is a gram-positive, rod-shaped bacterium that can cause listeriosis, a serious infection that affects people with weakened immune systems, pregnant women, and the elderly. The bacterium can contaminate various foods, particularly those with high moisture content, such as dairy products, meats, and vegetables.

**Why the Correct Answer is Right**
The correct answer is related to the pathophysiology of listeriosis. Listeria monocytogenes can invade host cells by exploiting the actin cytoskeleton, allowing it to spread intracellularly and cause infection. This process involves the activation of the small GTPase RhoA, which regulates actin polymerization and cell motility. The bacterium also produces two virulence factors, Listeriolysin O (LLO) and phospholipase C (PLC), which help it to escape from phagocytic vacuoles and survive within host cells.

**Why Each Wrong Option is Incorrect**
**Option A:** Listeria monocytogenes is typically transmitted through the fecal-oral route.
* This statement is incorrect because Listeria monocytogenes is primarily transmitted through the ingestion of contaminated food products, particularly those with high moisture content.

**Option B:** Listeria monocytogenes is a gram-negative bacterium.
* This statement is incorrect because Listeria monocytogenes is actually a gram-positive bacterium.

**Option C:** Listeria monocytogenes can cause listeriosis in immunocompetent individuals.
* This statement is incorrect because listeriosis typically affects people with weakened immune systems, pregnant women, and the elderly.

**Clinical Pearl / High-Yield Fact**
It is essential to note that Listeria monocytogenes can cause a range of clinical manifestations, including meningitis, encephalitis, and sepsis. Pregnant women are particularly vulnerable to listeriosis, which can lead to miscarriage, stillbirth, or preterm labor.

**Correct Answer: B. Listeria monocytogenes is a gram-negative bacterium.**