Genetic and biochemical characterisation of CTX-M-37 extended-spectrum β-lactamase from an Enterobacter cloacae clinical isolate from Mongolia

Abstract

OBJECTIVES: The aims of this study were to determine the resistance level of a blaCTX-M-37-carrying Enterobacter cloacae isolate from Mongolia, to analyse kinetic parameters of the purified enzyme and to compare the genetic environment of the gene.

METHODS: Minimum inhibitory concentrations (MICs) were determined using the Clinical and Laboratory Standards Institute (CLSI) agar dilution method. Purified CTX-M-37 enzyme was used to determined kinetic parameters. The genetic environment of the blaCTX-M-37 gene in E. cloacae was compared with a Kluyvera cryocrescens isolate.

RESULTS: The E. cloacae isolate showed relatively low-level resistance to cefotaxime (MIC=16mg/L) compared with a CTX-M-3-producing strain (MIC=256mg/L), and CTX-M-37 had a lower kcat/Km value for cefotaxime (2.0μM-1s-1) compared with CTX-M-3 (3.5μM-1s-1), possibly due to Asn114Asp substitution. The blaCTX-M-37 gene in the E. cloacae isolate was carried on a conjugative plasmid and was associated with an ISEcp1 element containing the -35 and -10 putative promoter sequences TTGAAA and TACAAT, respectively, unlike in the K. cryocrescens isolate.

CONCLUSIONS: The CTX-M-37-producing E. cloacae isolate showed relatively low-level resistance to cefotaxime and the purified enzyme had lower kinetic parameters as the result of Asn114Asp substitution. Presence of an ISEcp1 element and putative promoters upstream of the blaCTX-M-37 gene in E. cloacae, but not in the K. cryocrescens isolate, indicated their roles in mobilisation and expression of the gene.