CTX-M-190, a Novel β-Lactamase Resistant to Tazobactam and Sulbactam, Identified in an Escherichia coli Clinical Isolate.

A novel β-lactamase, CTX-M-190, derived from CTX-M-55 by a single substitution of Ser for Thr at position 133 (Ser133Thr), was identified in a natural Escherichia coli clinical isolate. CTX-M-190 exhibited potent hydrolytic activity against cefotaxime, with a kcat/Km ratio of 14.5 μM-1 s-1, and was highly resistant to inhibition by the β-lactamase inhibitors tazobactam and sulbactam, whose 50% inhibitory concentrations were 77- and 55-fold higher, respectively, for CTX-M-190 than for CTX-M-55. blaCTX-M-190 was located within the genetic platform ISEcp1-blaCTX-M-orf477, which was harbored by a 70-kb IncI1 plasmid.