Pleomorphic gram-positive bacilli in a Gram stain best describes which organism?

Pleomorphic gram-positive bacilli in a Gram stain best describes which organism?

• A. Bacillus anthracis
• B. Bacillus subtilis
• C. Listeria monocytogenes
• D. Corynebacterium pseudodiphtheriticum

Answer: (D)

Pleomorphism refers to bacteria that don’t keep one fixed shape; some Gram-positive bacilli show irregular, club-shaped forms and may arrange in palisades, giving a variegated, sometimes “Chinese character” appearance on Gram stain. Corynebacterium species are well known for this variability, often described as pleomorphic Gram-positive bacilli. Among the options, Corynebacterium pseudodiphtheriticum fits this description best because it belongs to the Corynebacterium group and can display pleomorphic shapes under Gram staining. The other organisms are typically more uniform in shape: Bacillus species are usually straight rods, and while Listeria monocytogenes can appear as small rods or coccobacilli, it is not classically described as pleomorphic.

Pleomorphism refers to bacteria that don’t keep one fixed shape; some Gram-positive bacilli show irregular, club-shaped forms and may arrange in palisades, giving a variegated, sometimes “Chinese character” appearance on Gram stain. Corynebacterium species are well known for this variability, often described as pleomorphic Gram-positive bacilli. Among the options, Corynebacterium pseudodiphtheriticum fits this description best because it belongs to the Corynebacterium group and can display pleomorphic shapes under Gram staining. The other organisms are typically more uniform in shape: Bacillus species are usually straight rods, and while Listeria monocytogenes can appear as small rods or coccobacilli, it is not classically described as pleomorphic.