This study was designed to depict prevalence and antimicrobial resistance characteristics of Proteus mirabilis (P. mirabilis) strains in 4 chicken farms and to probe the transfer mechanism of resistance genes. A total of 187 P. mirabilis isolates were isolated from 4 chicken farms. The susceptibility testing of these isolates to 14 antimicrobials showed that the multidrug resistance (MDR) rate was as high as 100%. The β-lactamase resistance genes bla_OXA-1, bla_CTX-M-1G, bla_CTX-M-9G and colistin resistance gene mcr-1 were highly carried in the P. mirabilis isolates. An MDR strain W47 was selected for whole genome sequencing (WGS) and conjugation experiment. The results showed that W47 carried 23 resistance genes and 64 virulence genes, and an SXT/R391 integrated conjugative elements (ICEs) named ICEPmiChn5 carrying 17 genes was identified in chromosome. ICEPmiChn5 was able to be excised from the chromosome of W47 forming a circular intermediate, but repeated conjugation experiments were unsuccessful. Among 187 P. mirabilis isolates, 144 (77.01%, 144/187) isolates carried ICEPmiChn5-like ICEs, suggesting that ICEs may be the major vector for the transmission of resistance genes among MDR chicken P. mirabilis strains in this study. The findings were conducive to insight into the resistance mechanism of chicken P. mirabilis strains and provide a theoretical basis for the use of antibiotics for the treatment of MDR P. mirabilis infections in veterinary clinic.

The increasing resistance to β-lactams and aminoglycoside antibiotics, mainly due to extended-spectrum β-lactamases (ESBLs) and 16S rRNA methylase genes, is becoming a serious problem in Gram-negative bacteria. Plasmids, as the vehicles for resistance gene capture and horizontal gene transfer, serve a key role in terms of antibiotic resistance emergence and transmission.

The emergence and spread of the multidrug-resistant gene cfr have raised significant public health concerns worldwide. To investigate its prevalence and dissemination dynamics, 18 cfr-positive strains were isolated in 2021 from fecal and environmental samples. Antimicrobial susceptibility testing showed that all strains were 100 % multidrug-resistant. Conjugation experiments demonstrated that a cfr- carrying IncFII(K)-IncR-IncFIB multi-replicon plasmid could transfer to E. coli J53. S1-nuclease digestion and Southern blotting identified cfr on plasmids of varying sizes, while whole-genome sequencing confirmed its presence on multiple plasmid types: IncX4, IncN, IncFII(K)-IncR-IncFIB, IncFIB-IncFII-IncR-IncHI2-IncHI2A multi-replicon plasmids, and two plasmids of unknown types. Genetic environment analysis revealed that cfr is categorized into five distinct structures (Types I-V). Reverse PCR results showed that Types I, II, and IV can form three circular intermediates of varying lengths (cfr-IS26). Network analysis further indicated strong association between cfr, tet(M), and dfrA14 mediated by IS26. Phylogenetic analysis revealed that the four ST1140 E. coli strains and all nine K. pneumoniae strains showed minimal genetic divergence. These findings suggest both clonal and horizontal transmission of cfr within the poultry farm. Continuous monitoring of cfr in animal-related environments is essential to mitigate its potential transfer to humans.