MICROEVOLUTION WITHIN A CLONAL POPULATION OF PATHOGENIC BACTERIA - RECOMBINATION, GENE DUPLICATION AND HORIZONTAL GENETIC EXCHANGE IN THE OPA GENE FAMILY OF NEISSERIA-MENINGITIDIS

Opacity (Opa) proteins are a family of antigenically variable outer-me mbrane proteins of Neisseria meningitidis. Even among clonally related epidemic meningococcal isolates, there is greater variation of Opa pr otein expression than can be accounted for by the cpa gene repertoire of any individual strain. We characterized the opa genes of eight clos ely related isolates of serogroup A N. meningitidis (subgroup IV-1) fr om a recent meningitis epidemic in West Africa. DNA sequence analysis and Southern blot experiments indicated that changes occurred in the o pa genes of these bacteria as they spread through the human population , over a relatively short period of time. Such changes in one or a few loci within a clonal population are referred to as microevolution. Th e distribution of sequences present in hypervariable (HV) regions of t he cpa genes suggests that duplication of all or part of cpa genes int o other cpa loci changed the repertoire of Opa proteins that could be expressed. Additional variability in this gene family appears to have been introduced by horizontal exchange of cpa sequences from other men ingococcal strains and from Neisseria gonorrhoeae. These results indic ate that processes of recombination and genetic exchange contributed t o variability in major surface antigens of this clonal population of p athogenic bacteria.