All of the following genera are oxidase-positive except which one?

Haemophilus is the genus that is not oxidase-positive. In microbiology, the oxidase test is used to determine the presence of cytochrome c oxidase in bacteria, which indicates their ability to utilize oxygen in respiration.

Neisseria, Pseudomonas, and Moraxella are all genera that contain species capable of producing cytochrome c oxidase, thus testing positive in the oxidase test. In contrast, Haemophilus species, while they are typically fastidious and require specific growth factors, do not produce this enzyme and therefore do not show a positive result on the oxidase test. This characteristic helps differentiate them from other gram-negative organisms that may be present in clinical samples or cultured in the laboratory.