Which organism is a Gram-positive, endospore-forming bacterium that fits the description of Clostridia?

• A. Clostridium tetani
• B. Clostridium botulinum
• C. Bacillus cereus
• D. Staphylococcus aureus

Answer: (A)

Clostridia are Gram-positive, endospore-forming, obligate anaerobes, so the word picture you should look for is a Gram-positive rod that can form endospores and grows without oxygen. Clostridium tetani fits this description perfectly: it is a Gram-positive rod that forms endospores and is an obligate anaerobe, a classic example used to illustrate clostridial organisms. This combination of traits is what makes it the best pick for representing the Clostridia group.

Bacillus cereus also forms endospores and is Gram-positive, but it belongs to a different genus (Bacillus) and is not a member of the Clostridia. Staphylococcus aureus is Gram-positive as well but does not form endospores, so it doesn’t match the defining feature. While another Clostridium in this list would also fit, the tetani example most clearly embodies the Clostridia characteristics being tested.