Which Bacillus species is most commonly isolated in opportunistic infections such as bacteremia and endocarditis?

• A. circulans
• B. cereus
• C. licheniformis
• D. subtilis

Answer: (B)

Bacillus cereus is the species most often seen in opportunistic infections like bacteremia and endocarditis. In people with weakened immune systems or those who have invasive devices or procedures, B. cereus can behave as a true pathogen rather than just an environmental contaminant. It’s more capable of causing invasive disease than the other Bacillus species listed, which are generally environmental and rarely cause such infections. While B. cereus is also known for food poisoning, its role as a cause of bloodstream infections and endocarditis in clinical settings is well established, making it the most likely culprit in these opportunistic infections.