In vitro effect of an antimicrobial combination therapy without colistin and tigecycline for CPE and non-CPE.

INTRODUCTION

This study aimed to investigate the in vitro effects of a combination of antimicrobials other than colistin (CL) and tigecycline (TGC) on carbapenem-resistant Enterobacteriaceae (CRE).

METHODS

We used 72 CRE strains including 65 carbapenemase-producing Enterobacteriaceae (CPE) that produce IMP-1, IMP-6, NDM, KPC, and OXA-48-like carbapenemases; and 7 carbapenemase-nonproducing Enterobacteriaceae (non-CPE) strains. These strains were assessed using antimicrobial susceptibility testing, breakpoint checkerboard (BC) plate method, and kill curve experiment to determine the effect of the combination therapy.

RESULTS

NDM, KPC, and OXA-48-like carbapenemase-producers showed higher MICs of carbapenem and aminoglycosides, and lower MICs of minocycline, compared to non-CPE and IMP-1/-6-producers. The results of the BC plate method suggested that the suitability of combinations of antimicrobials differ depending on the type of carbapenemases. Killing curve experiments demonstrated bactericidal or bacteriostatic action of the combination of antimicrobials even in sub-MIC concentrations of drugs. Our results suggest that the most effective antimicrobial combinations for each carbapenemase-producers are as follows; IMP-1 (tobramycin + tazobactam/piperacillin), IMP-6 (gentamicin + meropenem), NDM (minocycline + biapenem), KPC (arbekacin + doripenem) and OXA-48-like (minocycline + imipenem).

CONCLUSION

These results suggest that the combination of antimicrobials other than CL and TGC may be another candidate for the treatments of CPE infections, even though we have to choose effective antimicrobial combinations depending on the type of carbapenemase.