The Genomic View of Bacterial Diversification

Bacteria have unusual and variable sex lives, ranging from near-celibacy to evident promiscuity. “Sex” in bacteria involves a recipient bacterium replacing, by homologous recombination, small regions of its genome with corresponding regions from other strains ( 1 ). Studies over the past 20 years have demonstrated that in relatively promiscuous bacterial species, frequent recombination can drive rapid diversification of strains. In more celibate species, diversification is much slower and depends on the slow accumulation of point mutations ( 1 , 2 ). Sorting out the roles that these processes play in the pace and pattern of bacterial evolution, however, has proven problematic. On page 430 of this issue, Croucher et al. ( 3 ) demonstrate a powerful new approach to the problem. By comparing the genomes of many isolates of a single strain of a bacterium that causes pneumonia, they were able to rapidly obtain a comprehensive evaluation of the role that recombination, point mutation and other genetic processes played in its diversification. The approach offers insight into how this pathogenic strain can rapidly evolve resistance to antibiotics and evade future vaccines, and promises to help researchers better understand how this resistant strain may have spread globally.