The effect of triclosan on intergeneric horizontal transmission of plasmid-mediated tigecycline resistance gene tet(X4) from Citrobacter freundii isolated from grass carp gut.

The transmission of antibiotic resistance genes (ARGs) in pathogenic bacteria affects culture animal health, endangers food safety, and thus gravely threatens public health. However, information about the effect of disinfectants - triclosan (TCS) on ARGs dissemination of bacterial pathogens in aquatic animals is still limited. One Citrobacter freundii (C. freundii) strain harboring tet(X4)-resistant plasmid was isolated from farmed grass carp guts, and subsequently conjugative transfer frequency from C. freundii to Escherichia coli C600 (E. coli C600) was analyzed under different mating time, temperature, and ratio. The effect of different concentrations of TCS (0.02, 0.2, 2, 20, 200 and 2000 μg/L) on the conjugative transfer was detected. The optimum conditions for conjugative transfer were at 37 °C for 8h with mating ratio of 2:1 or 1:1 (C. freundii: E. coli C600). The conjugative transfer frequency was significantly promoted under TCS treatment and reached the maximum value under 2.00 μg/L TCS with 18.39 times that of the control group. Reactive oxygen species (ROS), superoxide dismutase (SOD) and catalase (CAT) activities, cell membrane permeability of C. freundii and E. coli C600 were obviously increased under TCS stress. Scanning electron microscope showed that the cell membrane surface of the conjugative strains was wrinkled and pitted, even broken at 2.00 μg/L TCS, while lysed or even ruptured at 200.00 μg/L TCS. In addition, TCS up-regulated expression levels of oxidative stress genes (katE, hemF, bcp, hemA, katG, ahpF, and ahpC) and cell membrane-related genes (fimC, bamE and ompA) of donor and recipient bacteria. Gene Ontology (GO) enrichment demonstrated significant changes in categories relevant to pilus, porin activity, transmembrane transporter activity, transferase activity, hydrolase activity, material transport and metabolism. Taken together, a tet(X4)-resistant plasmid could horizontal transmission among different pathogens, while TCS can promote the propagation of the resistant plasmid.