Evolutionary mapping of the SHV b-lactamase and evidence for two separate IS26-dependent blaSHV mobilization events from the Klebsiella pneumoniae chromosome

Results: Two main branches of the blaSHV evolutionary tree were located; both are derived from variant blaSHV-1 alleles. Identical mutations, responsible for extended-spectrum SHV substrate profiles, have been selected independently in each branch. There is evidence for a pool of non-mobile blaSHV framework sequences. Analysis of the genome sequence of Klebsiella pneumoniae confirms the chromosomal origin of blaSHV, whose mobilization has occurred at least twice, once for each of the main evolutionary branches. Both these mobilization events have been catalysed by IS26. Evolution of blaSHV to give common extended-spectrum variants is most likely to have occurred following mobilization.