A boxcar-shaped, gram-positive rod is most characteristic of which organism?

A boxcar-shaped, gram-positive rod is most characteristic of which organism?

• A. Clostridium difficile
• B. Corynebacterium diphtheriae
• C. Listeria monocytogenes
• D. Bacillus anthracis

Answer: (D)

Recognizing distinctive bacterial shapes helps ID organisms. A boxcar-shaped gram-positive rod describes straight rods with square ends that look rectangular under the microscope. This classic appearance is most characteristic of Bacillus anthracis, a large, non-motile, spore-forming bacillus that often presents with a poly-D-glutamic acid capsule and potent toxins (edema factor and lethal factor) contributing to disease. The other organisms have different characteristic features: Clostridium difficile is an anaerobic, spore-forming rod linked to antibiotic-associated colitis and isn’t defined by a boxcar shape; Corynebacterium diphtheriae shows club-shaped rods in a palisading arrangement; Listeria monocytogenes is a small rod with tumbling motility and intracellular growth, not noted for boxcar morphology.

Recognizing distinctive bacterial shapes helps ID organisms. A boxcar-shaped gram-positive rod describes straight rods with square ends that look rectangular under the microscope. This classic appearance is most characteristic of Bacillus anthracis, a large, non-motile, spore-forming bacillus that often presents with a poly-D-glutamic acid capsule and potent toxins (edema factor and lethal factor) contributing to disease. The other organisms have different characteristic features: Clostridium difficile is an anaerobic, spore-forming rod linked to antibiotic-associated colitis and isn’t defined by a boxcar shape; Corynebacterium diphtheriae shows club-shaped rods in a palisading arrangement; Listeria monocytogenes is a small rod with tumbling motility and intracellular growth, not noted for boxcar morphology.