An important factor in the evolution of bacteria is the ability to exchange DNA between different species by a variety of mechanisms, which holds profound implications for the development of antibiotic resistance. Majewski et al. have found that the ease of genetic exchange between individuals of the human pathogen Streptococcus pneumoniae and other strains and species depends on the relatedness, as measured by sequence divergence, of the DNA donor and recipient. If the DNA strands attempting to recombine differ in sequence, two factors appear to act to hinder recombination: the bacterial DNA editing machinery- the mismatch repair system, which will remove an entire doner strand- plus a reluctance to form DNA heteroduplexes because of a scarcity of regions of sequence identity between the donor and recipient strands. Hence, sexual isolation and speciation can still be achieved in bacteria despite their promiscuity.

But worringly, the mismatch repair system of S. pneumoniae appears to become overwhelmed by multiply mismatched DNA sequence. Related species could thus become important donor in the the evolution of the S. pneumoniae genome and may pose a problem when the tragets of drugs or vaccines are exposed to strong selective pressure, as summarized by Calverys et al.

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