N. meningitidis expresses two major porins, PorA and PorB, which are antigenically variable between strains and within a strain, and PorA is phase variable (random on/off switching). Neisseria gonorrhoeae expresses a single porin, PorB. Changes in porin expression or variant porins mediate antibiotic resistance in several Gram-negative bacteria, including N. gonorrhoeae.