Isolation, identification, and genomic features of Proteus mirabilis from diseased Asian arowana Scleropages formosus.

Proteus mirabilis is a Gram-negative opportunistic pathogen increasingly reported in aquatic animals; however, its pathogenic potential and genomic features in ornamental fish remain unclear. This study isolated 3 strains (WSY-112, WSY-113, and WSY-114) from diseased Asian arowana Scleropages formosus displaying hemorrhagic and ulcerative lesions. All 3 isolates were identified as P. mirabilis through physiological and biochemical characterization as well as 16S rRNA and gyrB sequencing. Strain WSY-114 was selected for further analyses, including whole-genome sequencing, antimicrobial susceptibility testing, and experimental infection. Genomic annotation identified multiple virulence-related genes for flagellar assembly, adhesion, toxin production, iron acquisition, and type III/IV/VI secretion systems. Experimental infection in zebrafish Danio rerio showed dose-dependent mortality (LD50 = 6.6 × 104 CFU g-1) and clinical signs mirroring natural infections in the arowana. The strain exhibited resistance to multiple tested antibiotics, including tetracyclines (tetracycline, minocycline, and doxycycline), macrolides (erythromycin and azithromycin), and ampicillin, consistent with antibiotic-resistance genes detected in its genome. These results indicate that P. mirabilis exhibits pathogenic potential under experimental conditions and may represent a potential risk factor in ornamental fish.