Which genus is a Gram-positive bacillus with central spores commonly discussed in anaerobic infection contexts?

• A. Bacillus
• B. Clostridium
• C. Lactobacillus
• D. Prevotella

Answer: (B)

The key idea is recognizing an organism that thrives without oxygen, forms endospores, and is classically tied to anaerobic infections. Clostridium are Gram-positive bacilli that are obligate anaerobes and produce endospores, which is why they are repeatedly encountered in discussions of anaerobic infections such as gas gangrene, botulism, tetanus, and antibiotic-associated colitis. Their ability to form spores helps them survive in harsh conditions and contribute to disease in low-oxygen environments. While spore position can vary among species (terminal or subterminal in many Clostridium), the critical point is their strict anaerobic metabolism and spore-forming capability, which sets them apart in this infectious context. In contrast, Bacillus are also spore-forming Gram-positive rods but are typically aerobic or facultative and not the archetypal anaerobic pathogens; Lactobacillus are non-spore-forming Gram-positive rods associated with normal flora and fermentation; Prevotella are Gram-negative anaerobes, not Gram-positive bacilli.