CTX-M-93, a CTX-M Variant Lacking Penicillin Hydrolytic Activity

Abstract

ABSTRACTExtended-spectrum β-lactamases (ESBLs) of the CTX-M type are increasingly being reported worldwide, with more than 90 known variants. ClinicalEscherichia coliisolate Bre-1 was isolated in 2009 and displayed an unusual ESBL phenotype, made of a synergy image between expanded cephalosporins and clavulanic acid discs and susceptibility to penicillins.E. coliBre-1 harbored a novel CTX-M-encoding gene, designatedblaCTX-M-93. CTX-M-93 differed from CTX-M-27 by only a single L169Q substitution. Compared to CTX-M-27, CTX-M-93 conferred higher MICs of ceftazidime forE. coli(MIC of 8 versus 1.5 μg/ml) and decreased MICs of other expanded-cephalosporins (MIC of cefotaxime of 1 versus 32 μg/ml) and penicillins (MIC of ticarcillin of 0.5 versus >256 μg/ml). A comparison of enzymatic properties revealed that the L169Q substitution led to a decreasedKmfor ceftazidime (25.5 versus 330 μM) but decreased hydrolytic activity against good substrates, such as cefotaxime (kcatof 0.6 versus 113 s−1), probably owing to the alteration of the omega loop positioning during the catalytic process. TheblaCTX-M-93gene was surrounded by the ISEcp1and IS903elements and inserted onto a 150-kb non-self-transferrable IncF-type plasmid.E. coliBre-1 belongs to phylogroup D and is of multilocus sequence type (MLST) 624, a sequence type found only in rare Spanish CTX-M-14-producingE. coliisolates. We have characterized a novel CTX-M variant, CTX-M-93, lacking significant penicillin hydrolysis but with increased ceftazidime hydrolysis.