Interaction of carbapenems and β-lactamase inhibitors towards CTX-M-15 and CTX-M-15G238C mutant

Loss and gain of ceftazidime-avibactam susceptibility in a non-carbapenemase-producing K1-ST23 hypervirulent Klebsiella pneumoniae

OBJECTIVES

This study aimed at revealing the underlying mechanisms of the loss and gain of ceftazidime-avibactam susceptibility in a non-carbapenemase-producing hypervirulent Klebsiella pneumoniae (hvKp).

METHODS

Here we longitudinally recovered 3 non-carbapenemase-producing K1-ST23 hvKp strains at a one-month interval (KP29105, KP29499 and KP30086) from an elderly male. Antimicrobial susceptibility testing, whole genome sequencing, transcriptomic sequencing, gene cloning, plasmid conjugation, quantitative real-time PCR (qRT-PCR), and SDS-PAGE (sodium dodecyl sulfate-polyacrylamide gel electrophoresis) were conducted.

RESULTS

Among the 3 hvKp strains, KP29105 was resistant to the third- and fourth-generation cephalosporins, KP29499 acquired resistance to both ceftazidime-avibactam and carbapenems, while KP30086 restored its susceptibility to ceftazidime-avibactam, imipenem and meropenem but retained low-level resistance to ertapenem. KP29105 and KP29499 carried plasmid-encoded genes blaCTX-M-15 and blaCTX-M-71, respectively, but KP30086 lost both. Cloning of gene blaCTX-M-71 and conjugation experiment of blaCTX-M-71-carrying plasmid showed that the transformant and transconjugant were susceptible to ceftazidime-avibactam but had a more than 8-fold increase in MICs. Supplementation with an outer membrane permeabilizer could reduce the MIC of ceftazidime-avibactam by 32 folds, indicating that porins play a key role in ceftazidime-avibactam resistance. The OmpK35 of the 3 isolates was not expressed, and the OmpK36 of KP29499 and KP30086 had a novel amino acid substitution (L359R). SDS-PAGE and qRT-PCR showed that the expression of porin OmpK36 of KP29499 and KP30086 was significantly down-regulated compared with KP29105.

CONCLUSIONS

In summary, we reported the rare ceftazidime-avibactam resistance in a non-carbapenemase-producing hvKp strain. Resistance plasmid carrying blaCTX-M-71 and mutated OmpK36 had a synergetic effect on the resistance.

Structural Comparisons of Cefotaximase (CTX-M-ase) Sub Family 1

The cefotaximase or CTX-M, family of serine-β-lactamases represents a significant clinical concern due to the ability for these enzymes to confer resistance to a broad array of β-lactam antibiotics an inhibitors. This behavior lends CTX-M-ases to be classified as extended spectrum β-lactamases (ESBL). Across the family of CTX-M-ases most closely related to CTX-M-1, the structures of CTX-M-15 with a library of different ligands have been solved and serve as the basis of comparison within this review. Herein we focus on the structural changes apparent in structures of CTX-M-15 in complex with diazabicyclooctane (DABCO) and boronic acid transition state analog inhibitors. Interactions between a positive surface patch near the active site and complementary functional groups of the bound inhibitor play key roles in the dictating the conformations of active site residues. The insights provided by analyzing structures of CTX-M-15 in complex with DABCO and boronic acid transition state analog inhibitors and analyzing existing structures of CTX-M-64 offer opportunities to move closer to making predictions as to how CTX-M-ases may interact with potential drug candidates, setting the stage for the further development of new antibiotics and β-lactamase inhibitors.