Neisseria (family Neisseriaceae) A genus of Gram-negative bacteria in which the cells are typically spherical and often occur in pairs with adjacent sides flattened. They are chemo-organotrophic. There are several species. They are parasitic on the mucous membranes of mammals. Some species are pathogenic: N. gonorrhoeae causes gonorrhoea, N. meningitidis causes meningitis. The organisms tend to be delicate and are easily killed by desiccation and by exposure to sunlight.

Neisseria (ny-seer-iă) n. a genus of spherical Gram-negative aerobic nonmotile bacteria characteristically grouped in pairs. N. gonorrhoeae (the gonococcus) the species that causes gonorrhoea, found within pus cells of urethral and vaginal discharge. N. meningitidis (the meningococcus) the species that causes meningococcal meningitis. Meningococci are found within pus cells of infected cerebrospinal fluid and blood or in the nasal passages of carriers.