Population genetics of a transformable bacterium: The influence of horizontal genetic exchange on the biology of Neisseria meningitidis

Abstract

Information of the biochemistry and genetics of bacterial species, usually obtained by the study of single isolates, is enhanced by studies of populations of bacteria. Recent advances in molecular technology, particularly polymerase chain reaction-based nucleotide sequence analysis, provide powerful tools for the study of population genetics. Data obtained by such techniques indicate that, while some bacterial species have a clonal population structure, others are non-clonal or panmictic. Clonal populations are a consequence of asexual reproduction by binary fission; panmictic population structures result from 'horizontal' exchange of genetic material between clones. A consequence of horizontal genetic exchange is mosaic gene structures, recognisable by comparisons of nucleotide sequences. In transformable bacteria, for example the human pathogen Neisseria meningitidis, several different genes, including the gene encoding the class 1 outer membrane protein, a major surface antigen, are mosaics. This genetic process has implications both for vaccine design and in the interpretation of epidemiological data.

The physical map of the chromosome of a serogroup A strain of Neisseria meningitidis shows complex rearrangements relative to the chromosomes of the two mapped strains of the closely related species N. gonorrhoeae

“…Although the gonococcal maps for strains FA1090 and MS11 are nearly identical (11,26), other gonococcal strains differ in the arrangement of rRNA operons on the chromosome (24). The natural competence of both meningococci and gonococci for genetic transformation may contribute to variability in chromosome organization as well as to the formation of mosaic genes containing information from different sources (28,33,37,47,55,70,82,95). Although we have not yet identified the endpoints of the rearranged segments of gonococcal or meningococcal DNA or the mechanisms by which such rearrangements are generated, the propensity of pathogenic Neisseria species to undergo such rearrangements may have important consequences for the evolution and pathogenic potential of these bacteria.…”

“…Neisseria spp. are naturally competent for genetic transformation (12), and genetic exchange occurs between pathogenic and commensal Neisseria species, between gonococci and meningococci, and among meningococcal strains (28,33,37,47,55,70,82,95). The impact of frequent recombination and horizontal genetic exchange events on the organization of the gonococcal and meningococcal chromosomes is not known.…”

Neisseria meningitidis phenotypic markers and septicaemia, disease progress and case-fatality rate of meningococcal disease: a 20-year population-based historical follow-up study in a Danish county

“…Recently, MLEE has been replaced by DNA multilocus sequence typing (MLST) (Maiden et al, 1998;Enright & Spratt, 1999) as the reference standard for characterization of meningococci. Genetic recombination takes place within N. meningitidis, resulting in the emergence of new strains (Maiden, 1993;Maynard Smith et al, 1993). Furthermore, genetic changes occur among strains with any defined phenotype (Scholten et al, 1994), which may be accompanied by altered virulence.…”

Clonal Analysis of Neisseria meningitidis Serogroup B Strains in South Africa, 2002 to 2006: Emergence of New Clone ST-4240/6688

“…(10). This diversity is attributed to high rates of recombination and phase variation within the naturally transformable meningococcal population (33). However, despite the high degree of diversity among serogroup B isolates, the majority of isolates collected over the 5-year period could be grouped into clusters representative of MLST clones.…”