Dissemination of the (X)-Variant Genes from Layer Farms to Manure-Receiving Soil and Corresponding Lettuce.

The occurrence of high-level tigecycline resistance (X) variant genes represents a new transferable resistance crisis to food safety and human health. Here, we investigated the abundance of (X)-variant genes [(X), (X1) to (X6)] in 33 samples collected from layer manures, manured/un-manured soils, and corresponding lettuce from three provinces in China. The results showed the occurrence of (X)/(X2), (X3), and (X4) in 24 samples. The detection rate of (X)/(X2) (23/24) is higher than that of (X3) (7/24) and (X4) (2/24), and (X)/(X2) and (X3) were found to be enriched and more abundant in most manured soil and several lettuce samples from manured soils than that from manure samples. Twenty six tigecycline-resistant bacteria were isolated, and (X)-variant genes were found to be disseminated not only by bacterial clone spreading but also via multidrug resistance plasmids. The total concentrations of (X)-variant genes showed significantly positive correlations ( = 0.683, < 0.001) with IS. Two veterinary tetracyclines (tetracycline and oxytetracycline) and other classes of antimicrobials (enrofloxacin, azithromycin, thiamphenicol, and florfenicol) showed significant correlations with the total concentrations of (X)-variant genes ( = 0.35-0.516, < 0.05). The findings indicate the transmission of (X)-variant genes from layer manures to their receiving environmental soils and lettuce and highlight the contribution of veterinary antimicrobials to the spread of (X)-variant genes.