Clonal spread of serogroup A meningococci: a paradigm for the analysis of microevolution in bacteria.

Extensive epidemiological analyses of epidemics of meningococcal meningitis have resulted in large, well-defined strain collections which represent the local diversity and global spread of serogroup A bacteria. Several genes for cell surface proteins are conserved during spread, with a few exceptions: analysis of these exceptions has revealed some of the phenomena which can lead to microevolution. Microevolution is so rapid with serogroup A meningococci that several independent recombination events have been documented within the last few decades. In a few cases, the recombinant bacteria have become established by clonal replacement plus epidemic spread. Comparison with other bacteria indicates that serogroup A meningococci provide a number of advantages for analysis of microevolution.