Screening for double carbapenem therapies effective against multidrug resistant Enterobacteriaceae

Abstract: Multidrug resistant bacteria account for one of the biggest threats to human health in the future. One treatment option to be evaluated is antibiotic combination therapy. For this purpose, last- line carbapenems have been suggested, even when bacteria are resistant, e.g. via production of degrading enzymes called carbapenemases. This study aimed to analyze combinations of ertapenem, doripenem and meropenem at clinically relevant concentrations against strains of Escherichia coli and Klebsiella pneumoniae carrying at least one carbapenemase (NDM, KPC or OXA-48). The screening methods used were a relatively new time-lapse microscopy method (oCelloScope), spot tests and time-kill experiments. The oCelloScope uses algorithms to illustrate bacterial growth over time. Moreover, a spot test involves dilution and plating of oCelloScope samples at 24 h. In the time-kill assay, samples are plated from tube cultures at specific time points. Depending on the difference in growth between antibiotic combinations and the most potent monotherapy, the effect was classified as synergistic, antagonistic or indifferent. According to the spot test, synergistic effects were observed against 12/30 strains (mainly carrying OXA-48) with one or more antibiotic combinations. A possible cause for this pattern is the lower activity of OXA-48, leading to lower resistance levels. Synergies also existed against some of the highly resistant strains (carrying NDM or KPC), which can be of great importance for clinical treatment. Due to a higher resolution of the spot test, the oCelloScope was unable to detect some of the synergistic combinations that were detected with the spot test. For future studies, more strains should be tested with time-kill, since only three strains were tested without observing any synergistic effects.