Meningococci produce medium-sized smooth colonies on blood agar plates after overnight incubation. Carbon dioxide enhances growth, but is not required. Twelve serogroups have been defined on the basis of the antigenic specificity of a polysaccharide capsule. The most important disease-producing serogroups are A, B, C, W-135, and Y. In addition to the group polysaccharides, individual N. meningitidis strains may contain two distinct classes of pili and multiple classes of OMPs. Some OMPs, porins, and adherence proteins have structural and functional similarities to those found in gonococci (see Table 20 – 1). The function of other OMPs is unknown.