Methods: Three clinical isolates of K. pneumoniae harboring CTX-M-15 and one E. coli with SHV-12 were used. The susceptibility of each isolate to piperacillin was determined by broth dilution using escalating concentrations of tazobactam, relebactam and avibactam. Similar experiments were subsequently conducted with ceftazidime. The resulting minimum inhibitory concentrations (MICs) were mapped as response to inhibitor concentration using an inhibitory Emax model. The best-fit model parameters were compared for each isolate-inhibitor combination.
Results: In all scenarios, MIC reductions were observed in the presence of increasing inhibitor concentrations. The MIC reduction for each isolate was well fitted to inhibitor concentrations (r2 ≥ 95%). IC50 estimates reflected the sensitivity of the isolates to each inhibitor, while Imax captured the maximum extent of MIC reduction. With piperacillin, IC50 values ranged from 1.36-35.25µg/ml for tazobactam, 2.32-15.82µg/ml for relebactam and 0.62-2.37µg/ml for avibactam. Imax values were 4.75-6.99, 6.56-9.77 and 7.83-11.22 for tazobactam, relebactam and avibactam respectively. Similar trends in IC50 and Imax were observed with ceftazidime as the β-lactam.
Conclusion: We illustrated a simple structural model capable of comparing the performance of different inhibitors. This platform may be used to identify the optimal pairing of various β-lactams and β-lactamase inhibitors for individual isolates.
C. Byerly, None
V. Tam, European Unionâs Seventh Framework Programme: Grant Investigator , Research grant .