Which genus comprises intracellular pathogens that are notable causes of zoonotic infections?

• A. Brucella
• B. Salmonella
• C. Vibrio
• D. Campylobacter

Answer: (A)

The main idea is that some bacteria live inside host cells, especially macrophages, which lets them persist and spread, and some of these intracellular pathogens are notable zoonotic threats. Brucella fits this pattern: it are gram-negative coccobacilli that infect livestock and survive and replicate within macrophages, avoiding destruction in the phagolysosome. This intracellular lifestyle underpins the chronic, flu-like illness known as brucellosis in humans, acquired through direct contact with infected animals or consumption of unpasteurized dairy products. Among the options, Brucella is the classic intracellular zoonotic pathogen, whereas the others are mainly extracellular pathogens or do not rely on an intracellular niche as their defining trait.