Sporadic Dissemination of tet(X3) and tet(X6) Mediated by Highly Diverse Plasmidomes among Livestock-Associated Acinetobacter

The emergence of high-level tigecycline resistance mediated by plasmid-borne tet(X) genes greatly threatens the clinical effectiveness of tigecycline. However, the dissemination pattern of plasmid-borne tet(X) genes remains unclear. We here recovered tet(X)-positive Acinetobacter isolates from 684 fecal and environmental samples collected at six livestock farms. Fifteen tet(X)-positive Acinetobacter isolates were identified, mainly including 9 tet(X3)- and 5 tet(X6)-positive Acinetobacter towneri isolates. A clonal dissemination of tet(X3)-positive A. towneri was detected in a swine farm, while the tet(X6)-positive A. towneri isolates mainly disseminated sporadically in the same farm. A tet(X3)-carrying plasmid (pAT181) was self-transmissible from a tigecycline-susceptible A. towneri strain to Acinetobacter baumannii strain ATCC 17978, causing 64- to 512-fold increases in the MIC values of tetracyclines (including tigecycline). Worrisomely, pAT181 was stably maintained and increased the growth rate of strain ATCC 17978. Further identification of tet(X) genes in 10,680 Acinetobacter genomes retrieved from GenBank revealed that tet(X3) (n = 249), tet(X5)-like (n = 61), and tet(X6) (n = 53) were the prevalent alleles mainly carried by four species, and most of them were livestock associated. Phylogenetic analysis showed that most of the tet(X3)- and tet(X6)-positive isolates disseminated sporadically. The structures of the tet(X3), and tet(X6) plasmidomes were highly diverse, and no epidemic plasmids were detected. However, cross-species and cross-region transmissions of tet(X3) might have been mediated by several plasmids in a small proportion of strains. Our study implies that horizontal plasmid transfer may be insignificant for the current dissemination of tet(X3) and tet(X6) in Acinetobacter strains. Continuous surveillance for tet(X) genes in the context of One Health is necessary to prevent them from transmitting to humans. IMPORTANCE Recently identified plasmid-borne tet(X) genes have greatly challenged the efficiency of tigecycline, a last-resort antibiotic for severe infection, while the dissemination pattern of the plasmid-borne tet(X) genes remains unclear. In this study, we identified a clonal dissemination of tet(X3)-positive A. towneri isolates on a swine farm, while the tet(X6)-positive A. towneri strains mainly disseminated sporadically on the same farm. Of more concern, a tet(X3)-carrying plasmid was found to be self-transmissible, resulting in enhanced tigecycline resistance and growth rate of the recipient. Further exploration of a global data set of tet(X)-positive Acinetobacter genomes retrieved from GenBank revealed that most of the tet(X3)- and tet(X6)-positive isolates shared a highly distant relationship, and the structures of tet(X3) and tet(X6) plasmidomes exhibited high mosaicism. Notably, some of the isolates belong to Acinetobacter species that are opportunistic pathogens and have been identified as sources of nosocomial infections, raising concerns about transmission to humans in the future. Our study evidenced the sporadic dissemination of tet(X3) and tet(X6) in Acinetobacter strains and the necessity of continuous surveillance for tet(X) genes in the context of One Health.