Clonal properties of meningococci from epidemic meningitis.

Methods from the field of population genetics now enable the classification of epidemic strains of Neisseria meningitidis and have resolved the relationships between apparently distinct epidemics. The diversity of serogroup A bacteria seems quite limited and only a few strains have been responsible for the epidemics of recent decades. Meningococci express both constant and highly variable antigens, the variability of which is determined by the clonal background of the epidemic strain. The development of an improved vaccine is being pursued but still faces technical problems.