A tissue Gram stain shows Gram-positive bacilli with central spores; after 24 hours, there is no growth on aerobic culture or on chocolate agar incubated with increased CO2. Which genus is most likely involved?

A tissue Gram stain shows Gram-positive bacilli with central spores; after 24 hours, there is no growth on aerobic culture or on chocolate agar incubated with increased CO2. Which genus is most likely involved?

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

Answer: (B)

A Gram-positive, spore-forming bacillus that cannot grow in the presence of oxygen points to an obligate anaerobe. Clostridium species are strict anaerobes and form spores, so they fit this growth pattern. The lack of growth on aerobic culture and on chocolate agar in increased CO2 indicates the organism cannot tolerate oxygen, which is characteristic of Clostridium. Other options don’t fit as well: Bacillus are typically aerobic or facultative and would grow under aerobic conditions or on enriched media; Lactobacillus lacks spores; Prevotella is Gram-negative. The key clue is a spore-forming Gram-positive rod that fails to grow in oxygen-rich conditions, making Clostridium the best match.

A Gram-positive, spore-forming bacillus that cannot grow in the presence of oxygen points to an obligate anaerobe. Clostridium species are strict anaerobes and form spores, so they fit this growth pattern. The lack of growth on aerobic culture and on chocolate agar in increased CO2 indicates the organism cannot tolerate oxygen, which is characteristic of Clostridium.

Other options don’t fit as well: Bacillus are typically aerobic or facultative and would grow under aerobic conditions or on enriched media; Lactobacillus lacks spores; Prevotella is Gram-negative. The key clue is a spore-forming Gram-positive rod that fails to grow in oxygen-rich conditions, making Clostridium the best match.