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

Objectives The aims of this study were to determine the resistance level of a bla _CTX-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 = 16 mg/L) compared with a CTX-M-3-producing strain (MIC = 256 mg/L), and CTX-M-37 had a lower k _cat /K _m value for cefotaxime (2.0 μM ⁻¹  s ⁻¹ ) compared with CTX-M-3 (3.5 μM ⁻¹  s ⁻¹ ), possibly due to Asn114Asp substitution. The bla _CTX-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 bla _CTX-M-37 gene in E. cloacae, but not in the K. cryocrescens isolate, indicated their roles in mobilisation and expression of the gene.