Plasmid-borne mcr-4.3 in Acinetobacter nosocomialis recovered from imported beef cattle: Genomic characterization and one health implications.

BACKGROUND

The emergence of mobile colistin resistance determinants in clinically relevant Acinetobacter species poses a growing challenge to antimicrobial stewardship and One Health surveillance. Although mcr-4.3 has been reported in Acinetobacter, its occurrence in imported food animals and its broader phylogenomic context remain poorly defined.

METHODS

We investigated fecal samples collected from 150 beef cattle imported from Australia to China. Acinetobacter isolates recovered by selective culture were subjected to whole-genome sequencing, and the mcr-4.3-positive isolate was further resolved by hybrid assembly using short- and long-read sequencing. Comparative plasmid analysis and global phylogenomic analysis were performed using publicly available A. nosocomialis genomes.

RESULTS

Among 54 Acinetobacter isolates recovered from imported cattle, one A. nosocomialis isolate, NB148, carried mcr-4.3. Complete genome sequencing showed that mcr-4.3 was located on a 28,233-bp plasmid, designated pNB148_mcr-4.3, which harbored plasmid maintenance-associated toxin-antitoxin systems and shared substantial backbone similarity with previously reported mcr-4.3-bearing plasmids from Acinetobacter species. The mcr-4.3 region was positioned downstream of a putative recombinase and a Tn3-family transposase, suggesting a conserved genetic framework for this resistance determinant. Global phylogenomic analysis incorporating 574 publicly available A. nosocomialis genomes placed NB148 within a diverse international population characterized by mixing across animal, environmental, and human sources. Among the global collection, 11 genomes carried mcr-4.3, of which 10 belonged to ST279, indicating that this lineage may represent an important genomic background for the persistence and international distribution of mcr-4.3-positive A. nosocomialis.

CONCLUSIONS

This study documents plasmid-borne mcr-4.3 in A. nosocomialis recovered from imported beef cattle and places this finding within a broader international One Health context. Our data expand the known ecological range of mcr-4.3-positive A. nosocomialis and support strengthened genomic surveillance at animal-human-environment interfaces, particularly in importation and border inspection settings.