ECOLOGICAL SEPARATION AND GENETIC ISOLATION OF NEISSERIA-GONORRHOEAE AND NEISSERIA-MENINGITIDIS

Background: Classifying bacteria into species is problematic. Most mic robiologists consider species to be groups of isolates that share some arbitrary degree of relatedness of biochemical or molecular (such as DNA sequence) features and that, ideally, are clearly delineated from all other groups of isolates. The main problem in applying to bacteria a biological concept of species based on the ability or inability of their genes to recombine, is that recombination appears to be rare in bacteria in nature, as indicated by the strong linkage disequilibrium between alleles found in most bacterial populations. However, there ar e some naturally transformable bacteria in which assortative recombina tion appears to be so frequent that alleles are in, or close to, linka ge equilibrium. For these recombining populations a biological concept of species might be applicable. Results: Populations of Neisseria gon orrhoeae and Neisseria meningitidis from Spain were analysed by multil ocus enzyme electrophoresis. The data indicate that assortative recomb ination occurs frequently within populations, but not between populati ons. Similarly, the sequences of two house-keeping genes show no evide nce of intragenic recombination between N. gonorrhoeae and N. meningit idis Conclusions: N gonorrhoeae and N. meningitidis represent extremel y closely related 'sexual' populations that appear to be genetically i solated in nature, and thus conform to the biological concept of speci es. The extreme uniformity of N. gonorrhoeae house-keeping genes sugge sts that this species may have arisen recently as a clone of Iv. menin gitidis that could colonize the genital tract. Ecological isolation - of populations that can colonize the genital tract from those that can colenize the nasopharynx - may have been an important component in sp eciation, leading to a lower frequency of recombination between specie s than within species.