Isolation, Identification, and Pathogenicity of Aeromonas veronii from Diseased Paracobitis variegatus

Aeromonas veronii (A. veronii) is ubiquitous in aquatic habitats and infects diverse aquatic animals, causing high mortality and heavy economic losses in aquaculture. In this study, we initially isolated a Gram-negative bacterium (Pv001) from the visceral organs of naturally diseased Paracobitis variegatus. Based on morphological, biochemical, and 16S rRNA and gyrB gene sequence analysis, the isolated strain was accurately determined to be A. veronii. Virulence gene screening showed that the strain Pv001 carried multiple virulence genes, including Acg, Act, Alt, Exu, Lip, and Omp. In vitro antimicrobial evaluations revealed that ceftriaxone, chloramphenicol, ciprofloxacin, and tetracycline displayed significant inhibitory effects on Pv001, while ampicillin, penicillin, and lincomycin exhibited no detectable inhibitory action. The artificial challenge of Paracobitis variegatus with the isolated strain Pv001 via intraperitoneal injection reproduced the clinical signs observed in the affected farm, yielding a calculated LD50 of 8.61 × 105 CFU/fish. Histopathological analysis of the diseased fish indicated extensive intestinal mucosal destruction, vacuolar degeneration of liver cells, and localized necrosis, as well as a considerable number of melano-macrophage centers (MMCs) aggregating in the spleen. This study provides a scientific foundation for the prevention and control of epidemics by revealing the pathogenic features of A. veronii illness in P. variegatus.