Interactions of Polymyxin B in Combination with Aztreonam, Minocycline, Meropenem, and Rifampin against Escherichia coli Producing NDM and OXA-48-Group Carbapenemases.

Carbapenemase-producing pose an increasing medical threat. Combination therapy is often used for severe infections; however, there is little evidence supporting the optimal selection of drugs. This study aimed to determine the effects of polymyxin B combinations against carbapenemase-producing Escherichia coli. The interactions of polymyxin B in combination with aztreonam, meropenem, minocycline or rifampin against 20 clinical isolates of NDM and OXA-48-group-producing E. coli were evaluated using time-lapse microscopy; 24-h samples were spotted on plates with and without 4× MIC polymyxin B for viable counts. Whole-genome sequencing was applied to identify resistance genes and mutations. Finally, potential associations between combination effects and bacterial genotypes were assessed using Fisher's exact test. Synergistic and bactericidal effects were observed with polymyxin B and minocycline against 11/20 strains and with polymyxin B and rifampin against 9/20 strains. The combinations of polymyxin B and aztreonam or meropenem showed synergy against 2/20 strains. Negligible resistance development against polymyxin B was detected. Synergy with polymyxin B and minocycline was associated with genes involved in efflux (presence of , wild-type , and the mutation H44Q) and lipopolysaccharide synthesis ( C27Y, mutations, and L323S). Synergy with polymyxin B and rifampin was associated with sequence variations in , which plays a role in lipid A modification. Polymyxin B in combination with minocycline or rifampin frequently showed positive interactions against NDM- and OXA-48-group-producing E. coli. Synergy was associated with genes encoding efflux and components of the bacterial outer membrane.