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

<h4>Background</h4>The emergence of mobile colistin resistance determinants in clinically relevant Acinetobacter species poses a growing challenge to antimicrobial stewardship and One Health surveillance. Although <i>mcr-4.3</i> has been reported in <i>Acinetobacter</i>, its occurrence in imported food animals and its broader phylogenomic context remain poorly defined.<h4>Methods</h4>We investigated fecal samples collected from 150 beef cattle imported from Australia to China. <i>Acinetobacter</i> isolates recovered by selective culture were subjected to whole-genome sequencing, and the <i>mcr-4.3</i>-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 <i>A. nosocomialis</i> genomes.<h4>Results</h4>Among 54 <i>Acinetobacter</i> isolates recovered from imported cattle, one <i>A. nosocomialis</i> isolate, NB148, carried <i>mcr-4.3</i>. Complete genome sequencing showed that <i>mcr-4.3</i> 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 <i>mcr-4.3</i>-bearing plasmids from <i>Acinetobacter</i> species. The <i>mcr-4.3</i> region was positioned downstream of a putative recombinase and a Tn<i>3</i>-family transposase, suggesting a conserved genetic framework for this resistance determinant. Global phylogenomic analysis incorporating 574 publicly available <i>A. nosocomialis</i> genomes placed NB148 within a diverse international population characterized by mixing across animal, environmental, and human sources. Among the global collection, 11 genomes carried <i>mcr-4.3</i>, of which 10 belonged to ST279, indicating that this lineage may represent an important genomic background for the persistence and international distribution of <i>mcr-4.3</i>-positive <i>A. nosocomialis</i>.<h4>Conclusions</h4>This study documents plasmid-borne <i>mcr-4.3</i> in <i>A. nosocomialis</i> recovered from imported beef cattle and places this finding within a broader international One Health context. Our data expand the known ecological range of <i>mcr-4.3</i>-positive <i>A. nosocomialis</i> and support strengthened genomic surveillance at animal-human-environment interfaces, particularly in importation and border inspection settings.