Molecular and kinetic comparison of the novel extended-spectrum beta-lactamases CTX-M-25 and CTX-M-26

CTX-M-25 is a novel extended-spectrum β-lactamase isolated from a single Canadian Escherichia coli isolate. Susceptibility testing demonstrated that this enzyme confers resistance to both cefotaxime and ceftazidime, but the level of resistance was reduced with the addition of β-lactamase inhibitors. The bla_CTX-M-25 gene was detected on a 111-kb plasmid. It is a member of the CTX-M-8 group and has the closest amino acid identity (99%; three amino acid substitutions) with CTX-M-26. The bla_CTX-M-26 gene was detected on a 100-kb plasmid isolated from a Klebsiella pneumoniae strain from the United Kingdom, and plasmid profiling revealed that it showed some homology to the bla_CTX-M-25-harboring plasmid. Both CTX-M genes were located downstream of ISEcp1, although the copy upstream of bla_CTX-M-25 was disrupted by IS50-A. Comparative kinetic studies of recombinant CTX-M-25 and CTX-M-26 enzymes showed that CTX-M-25 has a higher level of ceftazidime hydrolysis (k_cat values, 33 and 0.005 s⁻¹ for CTX-M-25 and CTX-M-26, respectively).