Recombination within natural populations of pathogenic bacteria: Short-term empirical estimates and long-term phylogenetic consequences

The identification of clones within bacterial populations is often taken as evidence for a low rate of recombination, but the validity of this inferen ce is rarely examined. We have used statistical tests of congruence between gene trees to examine the extent and significance of recombination in six bacterial pathogens. For Neisseria meningitidis, Streptococcus pneumoniae, Streptococcos pyogenes, and Staphylococcus aureus, the congruence between t he maximum likelihood trees reconstructed using seven house-keeping genes w as in most cases no better than that between each tree and trees of random topology. The lack of congruence between gene trees in these four species, which include both naturally transformable and nontransformable species, is in three cases supported by high ratios of recombination to point mutation during clonal diversification (estimates of this parameter were not possib le for Strep. pyogenes). In contrast, gene trees constructed for Hemophilus influenzae and pathogenic isolates of Escherichia coli showed a higher deg ree of congruence, suggesting lower rates of recombination. The impact of r ecombination therefore varies between bacterial species but in many species is sufficient to obliterate the phylogenetic signal in gene trees.