Cell-wall-inhibiting antibiotic combinations with activity against multidrug-resistant Klebsiella pneumoniae and Escherichia coli

The increasing prevalence of hospital and community-acquired infections caused by multidrug-resistant (MDR) bacterial pathogens is rapidly limiting the options for effective antibiotic therapy. Systematic studies on combinations of already available antibiotics that could provide an effective treatment against MDR bacteria are needed. We tested combinations of antibiotics that target one important physiological function (peptidoglycan synthesis) at several steps, and studied Enterobacteriaceae (Klebsiella pneumoniae and Escherichia coli) for which multidrug resistance associated with ESBL-producing plasmids has become a major problem. To measure the effectiveness of antibiotics alone and in combination, we used checkerboard assays, static antibiotic concentration time-kill assays, and an improved in-vitro kinetic model that simulates human pharmacokinetics of multiple simultaneously administered antibiotics. The target strains included an MDR K. pneumoniae isolate responsible for a recent major hospital outbreak. A double combination (fosfomycin and aztreonam) and a triple combination (fosfomycin, aztreonam and mecillinam) were both highly effective in reducing bacterial populations in all assays, including the in vitro kinetic model. These combinations were effective even though each of the MDR strains was resistant to aztreonam alone. Our results provide an initial validation of the potential usefulness of a combination of antibiotics targeting peptidoglycan synthesis in the treatment of MDR Gram-negative bacteria. We suggest that a combination of fosfomycin with aztreonam could become a useful treatment option for such infections and should be further studied.