Which organism is described as intracellular pathogens in humans?

• A. Bacillus anthracis
• B. Brucella spp.
• C. Shigella sonnei
• D. Vibrio vulnificus

Answer: (B)

Intracellular pathogens are organisms that can live and multiply inside host cells, especially macrophages, which lets them hide from some immune defenses and persist in the body. Brucella species are classic facultative intracellular pathogens in humans. They are small gram-negative coccobacilli that invade and survive within macrophages and other phagocytes, often residing in reticuloendothelial tissues like the liver, spleen, and bone marrow. Inside these cells, they can avoid complete destruction and replicate, leading to a chronic, systemic infection known as brucellosis. This intracellular lifestyle is the hallmark that makes Brucella stand out as an intracellular pathogen.

In contrast, the other organisms listed are not defined by sustained intracellular replication. Bacillus anthracis is primarily an extracellular pathogen that causes disease through toxins and capsule formation. Shigella sonnei invades the intestinal mucosa and is highly invasive, but its disease pattern centers on epithelial invasion rather than persistent intracellular replication in macrophages. Vibrio vulnificus causes rapid, extracellular tissue destruction and septicemia.