Antimicrobial Resistance and Molecular Characteristics in Tigecycline-Resistant Escherichia coli Isolates from Broilers in Jinan City, China

The emergence of plasmid-mediated tigecycline resistance in food-producing animals poses a significant threat to global public health by compromising a last-resort antimicrobial. This study investigated the prevalence, antimicrobial susceptibility profiles, and molecular genetic characteristics of tigecycline-resistant Escherichia coli (E. coli) isolated from broiler cecal samples in the Jinan region of Shandong Province, China, between 2020 and 2024. Antimicrobial susceptibility testing showed that high-level resistance to colistin (87.5%, 14/16), florfenicol (81.3%, 13/15), and enrofloxacin (75.0%, 12/16) was observed. Notably, a single isolate was resistant to meropenem (6.3%, 1/16). Whole-genome sequencing and subsequent in silico analysis demonstrated that all 16 tigecycline-resistant isolates harbored the tigecycline resistance gene tet(X4), the primary determinant of resistance. Molecular typing identified a diverse population structure, with sequence type 224 (ST224) being the dominant clone, accounting for 37.5% (6/16) of the isolates. The genetic milieu of resistance was complex, characterized by the co-existence of tet(X4) with multiple other clinically important resistance genes, including the mobile colistin resistance gene mcr-1 (87.5%, 14/16) and various extended-spectrum β-lactamase genes such as blaCTX-M variants and, critically, blaNDM-5. Furthermore, an array of virulence factor genes was identified, with a particularly high prevalence of the toxin gene astA (68.8%, 11/16) and the bacteriocin gene cma (50.0%, 8/16), indicating pathogenic potential. This convergence of resistance and virulence in a foodborne pathogen highlights an urgent need for continuous One Health surveillance to mitigate the risks posed by these potentially untreatable “superbugs” to both animal and human health.