Microevolution during epidemic spread of Neisseria meningitidis.

Similar to many other naturally transformable bacteria, Neisseria meningitidis has yielded many examples where horizontal genetic exchange has resulted in genetic variation of individual strains. Epidemic strains are purified of genetic variants due to bottlenecks during the spread from country to country, resulting in clonal descent. Occasionally, clonal replacement also occurs during epidemic spread. These processes occur rapidly in serogroup A meningococci; such that after their descent from a common ancestor, clonally related bacteria have diversified at numerous loci within the last decades.