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 Bacillus species most often associated with true invasive disease in susceptible people, including bacteremia and endocarditis. In clinical settings, many Bacillus isolates are just contaminants, but B. cereus has the capacity to cause serious infections, particularly in patients with weakened immune systems or with implanted devices where it can form biofilms and seed the bloodstream or prosthetic valves. This makes it the most common Bacillus pathogen in these opportunistic infections. The other species listed are more commonly environmental organisms and are much less frequently implicated in invasive disease, so they’re rarely the cause of bacteremia or endocarditis compared with cereus.