A Gram-positive bacillus with boxcar arrangement, catalase positive, non-motile. Which organism is correct?

Prepare for the CAMLPR Microbiology Test. Utilize flashcards and engaging multiple-choice questions, each equipped with descriptions and explanations. Ready yourself for an excellent performance!

Boxcar arrangement points to a very specific morphological pattern: long chains of Gram-positive bacilli with squared-off ends that resemble boxcars, a description commonly associated with Bacillus anthracis. The fact that the organism is catalase positive fits Bacillus species, but the non-motile trait is the key discriminator here, since many Bacillus strains are motile while Bacillus anthracis is not. Clostridium species are obligate anaerobes and typically catalase negative, so catalase positivity helps rule them out. Lactobacilli are usually catalase negative and don’t show boxcar-chain morphology. Taken together, the match is Bacillus anthracis.