A clonal analysis of Neisseria meningitidis serogroup A

A typing scheme has recently been developed for Neisseria meningitidis serogroup A based on the clonal population structure of these bacteria. An international strain collection consisting of 423 group A strains isolated from 23 epidemics or outbreaks since 1963, as well as from older epidemics and numerous non-epidemic situations was used in the analysis. Strains were first segregated into electrophoretic types, depending on the combined score for the electrophoretic mobilities of 7 cytoplasmic isoenzymes resolved by starch gel electrophoresis and of 2 outer membrane proteins resolved by sodium dodecyl sulphate polyacrylamide gel electrophoresis. The bacteria were subsequently assigned to one of 21 clones after numerical analysis of their electrophoretic types.The epidemiological value of the typing scheme was assessed by examining case and carrier strains isolated during (1982–83) and subsequent to (1984–85) an epidemic in the Gambia, West Africa. The case isolates, all of which were serogroup A, were of a single clonal type. All serogroup A carrier isolates were also of this clone, while carrier strains of other serogroups showed greater clonal diversity. These results indicate that case strains during an epidemic show little clonal diversity and thus that the typing scheme is of value in distinguishing the etiology of epidemics.A retrospective epidemiological analysis of the strains in the international collection showed that most serogroup A epidemics were associated with a single or predominant clone, although some epidemics were of mixed etiology. The survey included 256 isolates from 15 African epidemics since 1963, a period which covers 3 major epidemic waves (1960–63; 1967–73 and 1981–83), thus permitting a detailed epidemiological analysis of serogroup A epidemics in this continent.Epidemiological records indicate that seven clones have been responsible for sets of epidemics throughout the world since 1915 and that at least two of these sets can be considered to represent mutually exclusive pandemics, linking numerous epidemics between 1967–75 and 1973–83, respectively.