Novel tigecycline resistance mechanisms in mediated by mutations in , and .

is an important pathogen in hospital acquired infections. Although tigecycline currently remains a potent antibiotic for treating infections caused by multidrug resistant (MDRAB) strains, reports of tigecycline resistant isolates have substantially increased. The resistance mechanisms to tigecycline in are far more complicated and diverse than what has been described in the literature so far. Here, we characterize -selected MDRAB strains obtained by increasing concentrations of tigecycline. We have identified mutations in , and that result in reduced susceptibility to tigecycline. Using complementation experiments, we confirm that mutations in , , and two types of mutations in correlate with tigecycline resistance. By Western blot and polysome profile analysis, we demonstrate that the mutation results in decreased expression of RRF, which affects the process of ribosome recycling ultimately leading to increased tigecycline tolerance. A transcriptional analysis shows that the mutated gene plays a role in regulating the expression of the SAM-dependent methyltransferase () and transcriptional regulators, to confer moderate tigecycline resistance. This study provides direct evidence that mutations in the , and are associated with tigecycline resistance in .