A Gram-positive bacillus with boxcar arrangement, catalase positive, and motile. Which organism is most likely?

• A. Clostridium perfringens
• B. Bacillus spp
• C. Lactobacilli
• D. Staphylococcus aureus

Answer: (B)

Identifying motile, catalase-positive Gram-positive rods helps distinguish Bacillus from other bacteria. The boxcar arrangement describes long, uniform rods that can align in parallel in Gram stains, giving a boxlike appearance typical of Bacillus species. The catalase test shows bubbles when hydrogen peroxide is added, indicating the organism breaks down hydrogen peroxide—this is a common trait of Bacillus and other aerobic organisms, and helps separate them from anaerobic Clostridium species, which are usually catalase negative. Motility further supports Bacillus, since many Bacillus species are flagellated and motile, whereas Staphylococcus aureus is cocci (not a bacillus) and Lactobacilli are often non-motile and frequently catalase negative. Putting these clues together, a Gram-positive bacillus that is motile and catalase positive fits Bacillus species best.