Emergence of AcrAB-mediated tigecycline resistance in a clinical isolate of during ciprofloxacin treatment

Tigecycline resistance remains rare amongst Enterobacteriaceae in the UK, as elsewhere, but has been associated with upregulation of the AcrAB efflux system. Using isolates of an strain that developed tigecycline resistance in vivo during ciprofloxacin therapy as well as laboratory-selected mutants, we investigated the role of this pump and the global regulator RamA in tigecycline resistance. Laboratory mutants were selected from a susceptible clinical isolate in vitro by exposure to increasing concentrations of tigecycline. Expression of the operon and the gene was monitored by real-time reverse-transcription polymerase chain reaction (RT-PCR). Overexpression of was achieved using the pBAD expression vector, whilst insertional inactivation of with a gentamicin resistance cassette was achieved with the bacteriophage λ Red recombination system. Increased tigecycline minimum inhibitory concentrations in the clinical isolate and a laboratory mutant were associated with increases in and transcripts. Induction of increased expression resulted in increased expression, whilst insertional inactivation of restored full susceptibility to tigecycline. Treatment with ciprofloxacin, a substrate of AcrAB in , possibly selected for cross-resistance to tigecycline as a result of RamA-mediated AcrAB upregulation.