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Similar to other CTX-M family enzymes, KLUC is a recently identified and emerging determinant of cefotaxime resistance that has been recovered from at least three  Enterobacteriaceae  species, including  Kluyvera cryocrescens ,  Escherichia coli,  and  Enterobacter cloacae . Whether this extended-spectrum β-lactamase (ESBL) has been disseminated among commonly isolated  Enterobacteriaceae  is worthy of further investigation. In this study, we screened 739 nosocomial  Enterobacteriaceae  isolates (240  Klebsiella pneumoniae  and 499  E. coli  strains) and found that one  K. pneumoniae  and four  E. coli  isolates harbored the  bla  KLUC  gene. Three  bla  KLUC  determinants isolated from  E. coli  were entirely identical to a  bla  KLUC-3  gene previously recovered in the same hospital. PFGE of four  bla  KLUC -harboring  E. coli  strains showed that prevalence of these determinants was most likely mediated by horizontal gene transfer but not clonal dissemination. However, the variant isolated from  K. pneumoniae  belonged to a novel member of the KLUC enzyme group. This newly identified enzyme (KLUC-5) has an amino acid substitution compared with previously identified KLUC-1 (G18S) and KLUC-3 (G240D). Antimicrobial susceptibility tests showed that KLUC-5 significantly reduced resistance activity to almost all the selected antimicrobials compared to previously identified KLUC-3. Site-directed mutagenesis showed that  bla  KLUC-5 -D240G and  bla  KLUC-5 -S18G significantly enhanced the MIC against its best substrate. Conjugation and S1-PFGE indicated that  bla  KLUC-5  was located on a transferable plasmid, which was further decoded by single-molecule, real-time sequencing. Comparative genome analysis showed that its backbone exhibited genetic homology to the IncA/C incompatibility group plasmids. A transposable element, IS Ecp1 , was detected 256-bp upstream of the  bla  KLUC-5  gene; this location was inconsistent with the previously identified  bla  KLUC-1  but congruent with the variants recovered from  E. coli  in the same hospital. These data provide evidence of the increasingly emerging KLUC group of ESBLs in China.

Characterization of a Novel blaKLUC Variant With Reduced β-Lactam Resistance From an IncA/C Group Plasmid in a Clinical Klebsiella pneumoniae Isolate