Which of the following is a Gram-positive to Gram-variable coccobacillus?

Listeria monocytogenes is recognized as a Gram-positive to Gram-variable coccobacillus due to its unique cell wall structure and staining characteristics. In Gram staining, this bacterium typically retains the crystal violet dye, which characterizes Gram-positive organisms; however, it may show variable results depending on the age of the culture and the conditions in which it is grown, sometimes exhibiting characteristics similar to Gram-negative organisms.

Listeria is a coccobacillus, meaning it has a rod-like shape that is often somewhat rounded at the ends, giving it a coccoid appearance. This morphology is distinct and essential in differentiating it from other bacteria.

In contrast, the other options provided do not match these characteristics. Salmonella enterica and Shigella dysenteriae are Gram-negative bacilli, meaning they do not retain the crystal violet stain and are typically longer and appear more rod-shaped rather than coccobacillary. Vibrio cholerae is also a Gram-negative bacterium but is curved or comma-shaped rather than being classified as a coccobacillus. Therefore, Listeria monocytogenes stands out as the only accurate representation of a Gram-positive to Gram-variable coccobacillus among the choices