Which of the following is NOT gram-negative?

Peptococcus is correctly identified as the option that is not gram-negative because it is a member of the genus of bacteria that are classified as gram-positive. Gram-positive bacteria have a thick peptidoglycan layer in their cell wall, which retains the crystal violet stain used during the Gram staining procedure, resulting in a purple appearance under a microscope.

In contrast, Salmonella, Branhamella, and Aeromonas are all considered gram-negative, characterized by a thinner peptidoglycan layer and an outer membrane that does not retain the crystal violet stain. Instead, during the Gram staining process, these bacteria will take up the counterstain (usually safranin) and appear pink under the microscope. Thus, the clear distinction of Peptococcus as gram-positive makes it the correct answer to this question.