Effect of Porins and bla Expression on Activity of Imipenem with Relebactam in Klebsiella pneumoniae: Can Antibiotic Combinations Overcome Resistance?

Imipenem with relebactam is a novel β-lactam-β-lactamase inhibitor that has activity against most KPC-producing Enterobacteriaceae. Using 10 isolates of KPC-possessing Klebsiella pneumoniae, we assessed the relationship between imipenem-relebactam minimum inhibitory concentrations (MICs) and mechanisms known to contribute to antimicrobial resistance. The effect of adding a second agent was assessed by time-kill experiments. Mutations affecting the genes encoding porins ompK35 and ompK36 and identification of β-lactamases were assessed by PCR. Expression of bla and acrB was assessed by real-time reverse-transcriptase (RT)-PCR, and production of OmpK36 by SDS-PAGE. Time-kill studies were performed using combinations of imipenem-relebactam with polymyxin B, amikacin, or tigecycline. Seven isolates having a disruption in a single porin, or in neither porin, remained susceptible to imipenem-relebactam. The addition of a second agent did not improve the activity of imipenem-relebactam for these isolates, although the addition of tigecycline was antagonistic for three isolates. Three isolates had major disruptions in both ompK35 and ompK36 that correlated with reduced activity of imipenem-relebactam (MICs 2/4, 8/4, and 512/4 μg/mL). Two of these isolates also had overexpression of bla, including the isolate with the highest MIC. These isolates were also resistant to polymyxin B and amikacin. The addition of amikacin provided both synergistic and bactericidal activity for the two more resistant isolates. The activity of imipenem-relebactam against K. pneumoniae is affected by major disruptions of both ompK35 and ompK36 and by expression of the KPC gene. Combining imipenem-relebactam with an aminoglycoside may be a promising approach for isolates with reduced susceptibility to imipenem-relebactam.