Characterization of Fitness Cost Caused by Tigecycline-Resistance Gene (X6) in Different Host Bacteria.

The emergence and prevalence of the (X) gene and its variants in the environment and in clinical settings constitute a growing concern for public health worldwide. Accordingly, the tigecycline resistance gene variant (X6) is widely detected in spp. and spp. rather than Enterobacteriaceae, while the underpinning behind this phenomenon is still unclear. To investigate the mechanisms underlying this distinct phenomenon, we assessed the fitness of the engineered plasmid pBAD-(X6) in different host bacteria by monitoring their growth curves, relative fitness and the ability of biofilm formation, as well as virulence in a model. MIC and qRT-PCR analysis indicated the successful expression of the (X6) gene in these strains in the presence of l-arabinose. Furthermore, we found that pBAD-(X6) displayed the lowest fitness cost in compared with that in or Enteritidis, suggesting the fitness difference of (X6)-bearing plasmids in different host bacteria. Consistently, the carriage of pBAD-(X6) remarkably reduced the biofilm production and virulence of or Enteritidis. These findings not only indicate that the fitness cost difference elicited by the (X6) gene may be responsible for its selectivity in host bacteria but also sheds new insight into the dissemination of antibiotic resistance genes (ARGs) in clinical and environmental isolates.