Brucella spp. are which of the following?

• A. The etiologic agents of relapsing fever
• B. Small spiral organisms
• C. Primarily a cause of endogenous human infections
• D. Intracellular pathogens

Answer: (D)

Brucella species are intracellular pathogens. They are Gram-negative coccobacilli that invade and survive inside macrophages, multiplying within phagocytes and disseminating to organs like the liver, spleen, bone marrow, and lymph nodes. This intracellular lifestyle explains the chronic, systemic infection and sometimes undulating fever seen with brucellosis. They are not the agents of relapsing fever (that's Borrelia, spirochetes), nor are they simply small spiral organisms; and human Brucella infections are typically zoonotic, acquired from animals or animal products, not endogenous. So the description that they are intracellular pathogens best fits their biology and disease pattern.