Bovine-Derived Acinetobacter indicus Co-Harboring Chromosomal tet(X3) and Plasmid-Located tet(X4) Isolated from Henan, China

The coexistence of antibiotic resistance genes (ARGs), particularly those conferring resistance to last-resort antibiotics, is increasingly present in lesser-studied bacterial species. Tigecycline is currently one of the last important barriers in the treatment of carbapenem-resistant bacterial infections, whose resistance gene, tet(X), is prevalent across multiple bacterial genera, but the coexistence of tet(X3) and tet(X4) in Acinetobacter sp. is rarely observed. Here, we report a strain co-harboring the chromosomal tet(X3) and plasmid-borne tet(X4) isolated from a commercial beef cattle farm in Henan province, China. The strain exhibited resistance to ampicillin, gentamicin, chloramphenicol, sulfamethoxazole, tetracycline, doxycycline, tigecycline, and omadacycline. Based on whole-genome sequencing (WGS), the strain was identified as A. indicus using Average Nucleotide Identity (ANI) and digital DNA–DNA hybridization (dDDH). Chromosomal tet(X3) was identified in the genetic context, ISVsa3-XerD-tet(X3)-res-ISVsa3. The plasmid-located tet(X4) with the genetic context, ISVsa3-abh-tet(X4)-ISVsa3, and 14 additional resistance genes were located in multiple pdif modules. Two different typing methods, the Rep-based strategy (designed for A. baumanii) and MOB-typer, identified the tet(X4)-positive plasmid as GR31 and rep_cluster_1656, respectively. Conjugation assay failed to observe the transfer of the tet(X4)-positive plasmid into recipients, E. coli J53 and Salmonella LGJ2. The co-occurrence of tet(X3) and tet(X4) in Acinetobacter may suggest a risk of rapid dissemination of tigecycline resistance and the hidden presence of numerous undetected bacteria co-carrying high-risk ARGs in the agroecological system, both of which should cause particular concern.