Isolation, Identification, and Biological Characterization of Proteus mirabilis Isolated from Beef Cattle in China

Proteus mirabilis, a member of the family Enterobacteriaceae, is widely distributed in the environment and commonly colonizes the gastrointestinal tract of animals. Although traditionally regarded as an opportunistic zoonotic pathogen, increasing evidence suggests that it may also contribute to bovine diseases. However, information regarding the antimicrobial resistance, virulence determinants, and genomic characteristics of bovine-associated P. mirabilis in China remains limited. In this study, a P. mirabilis strain, designated A3, was isolated from one of 19 diarrheic adult beef cattle sampled in Anhui Province, China, and comprehensively characterized through phenotypic, genomic, and pathogenicity analyses. Strain A3 exhibited typical swarming motility and a multidrug-resistant phenotype. PCR screening identified the resistance genes aac(6′)-Ib and qnrA, together with seven virulence genes (pmfA, atfA, ureC, mrpA, ucaA, zapA, and atfC) involved in adhesion and colonization. Whole-genome sequencing generated a 3,965,086 bp draft genome with a GC content of 38.37% and identified 85 resistance-associated genes and 147 virulence-associated genes, including multidrug efflux pumps and virulence factors related to adhesion, motility, iron acquisition, secretion systems, and host interaction. Comparative genomic analysis demonstrated high genomic similarity to other bovine-derived P. mirabilis isolates, with average nucleotide identity (ANI) values ranging from 99.06% to 99.32%, while pangenome analysis identified 5680 orthologous gene clusters, including 2959 core genes, indicating an open pangenome structure. Multiple insertion sequences, prophages, and genomic islands were identified, highlighting substantial genomic plasticity and adaptive potential. In a mouse infection model, strain A3 exhibited clear dose-dependent pathogenicity. All eight mice challenged with 2.22 × 109 CFU/mL died within 12 h post-infection and developed severe pathological lesions in multiple organs, whereas only one of eight mice in the low-dose group died during the same period. Collectively, these findings indicate that bovine-derived P. mirabilis A3 combines multidrug resistance, genomic plasticity, and opportunistic pathogenic potential. This study expands current knowledge of bovine-associated P. mirabilis and provides a genomic and experimental basis for surveillance, risk assessment, and prevention strategies in cattle production systems.