Tazobactam selects for multidrug resistance.

Piperacillin-Tazobactam is a β-lactam/β-lactamase inhibitor combination that is amongst the most prescribed antimicrobials in hospital medicine. Piperacillin is inactivated by many common beta-lactamases, but tazobactam inhibits these, allowing successful treatment. The effect of piperacillin on Gram-negative bacteria has been widely studied, but less attention has been paid to the effects of tazobactam. We used a massive transposon mutagenesis approach (TraDIS-Xpress) to identify genes in Escherichia coli that affect survival when exposed to piperacillin and tazobactam, separately and together. We found significant differences between the two drugs: a striking finding was that multiple efflux pump families and regulators were essential for survival in the presence of tazobactam, but only one efflux system was beneficial for piperacillin. Exposure of Escherichia coli and Klebsiella pneumoniae to piperacillin and/or tazobactam selected for mutants with reduced susceptibility, and tazobactam selected for efflux and cell envelope mutants associated with multidrug resistance. We identified other pathways involved in tazobactam susceptibility, including the shikimate kinase AroK. Genes involved in DNA replication and repair reduced E. coli susceptibility to a combination of piperacillin and tazobactam but were not identified after exposure to either drug alone. Tazobactam can select for mutants with increased efflux activity, and the development of future β-lactamase inhibitors should consider potential selective impacts of both inhibitor and antibiotic.