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 repcluster₁656, 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.
Wang et al. (Thu,) studied this question.