Small clone dissemination of tmexCD1-toprJ1–carrying Klebsiella pneumoniae isolates in a chicken farm

Tigecycline is one of the last-resort treatments against multidrug-resistant (MDR) Gram-negative bacteria. Recently, a novel efflux pump gene cluster tmexCD1-toprJ1 has been identified to reduce the antibacterial activity of tigecycline. However, the epidemiological feature and the horizontal transfer of this cluster have not been thoroughly studied in chicken-origin strains. Here, we found that the tmexCD1-toprJ1 gene cluster was present in 32.7% (16/49) of tigecycline-resistant Klebsiella pneumoniae (K. pneumoniae) isolates in a chicken farm. All the strains showed MDR phenotype, and the efflux pump inhibitor 1-(1-naphthylmethyl)-piperazine (NMP) synergized the activity of tigecycline to the tmexCD1-toprJ1–carrying strains by 8- to 128-fold. The S1 nuclease–pulsed field gel electrophoresis (S1-PFGE), and Southern blotting revealed that all the tmexCD1-toprJ1 gene clusters were located on plasmids with various sizes and could be transferred among strains by conjugation and transformation. Furthermore, the tmexCD1-toprJ1–carrying plasmid could have a fitness cost in recipient strains. Sixteen isolates belonged to two different sequence types (ST3332 and ST37), and they were classified into two distinct lineages. Importantly, one plasmid was found to co-harbour tmexCD1-toprJ1 and mcr-8.5, which may pose a potential threat to public health. This study demonstrates the clonal dissemination in tmexCD1-toprJ1–carrying K. pneumoniae strains in the chicken farm.