CLONAL DESCENT AND MICROEVOLUTION OF NEISSERIA-MENINGITIDIS DURING 30YEARS OF EPIDEMIC SPREAD

Serogroup A meningococci of subgroups III, IV-1 and IV-2 are probably descended from a common ancestor that existed in the nineteenth centur y. The 10.5kb sequences spanning five distinct chromosomal loci, encod ing cell-surface antigens, a secreted protease or housekeeping genes a nd intergenic regions, were almost identical in strains of those subgr oups isolated in 1966, 1966 and 1917 respectively. During the subseque nt two to three decades, all of these loci varied as a result of mutat ion, translocation or import of DNA from unrelated neisseriae. Thus, m icroevolution occurs frequently in naturally transformable bacteria. M any variants were isolated only once or within a single geographical l ocation and disappeared thereafter. Other variants achieved genetic fi xation within months or a few years. The speed with which sequence Var iation is either eliminated or fixed may reflect sequential bottleneck s associated with epidemic spread and contrasts with the results of ph ylogenetic analyses from bacteria that do not cause epidemics.