Aeromonas veronii infection in cultured Oreochromis niloticus: prevalence, molecular and histopathological characterization correlated to water physicochemical characteristics, with the protective autochthonous probiotic

Abstract This study characterized the bacterial pathogen associated with summer mortality in cultured Nile tilapia ( Oreochromis niloticus ) between June and August 2024 across two different farms. As well as, the autochthonous probiotic was isolated as a preventive strategy. Phenotypic characteristics identified Aeromonas veronii and confirmed by molecular sequencing of the gyrB gene. Total prevalence was higher in farm 1 (65.19%) than in farm 2 (51.11%), with respective mortality rates of 38.84% and 25.25%. Prevalence rates peaked in August, which was correlated significantly ( P ≤ 0.05) with elevated water temperature, pH, ammonia, and salinity. Clinical signs included erratic swimming, skin hemorrhages, and corneal opacity, with necropsy including ascitic fluids, friable liver, and splenomegaly. Challenge assay confirmed LD50 of 2.7 × 10 7 CFU/mL A. veronii and 66.67% cumulative mortality. Histopathological alterations showed intermuscular edema, glomerular atrophy of the kidneys, vacuolar degeneration of the liver, spleen necrosis, and hemosiderosis of intestinal tissues. A. veronii showed resistance to ampicillin, tigecycline, fosfomycin, and sulfamethoxazole-trimethoprim. To control Aeromonas infection, 18 autochthonous probiotic isolates were obtained from the healthy O. niloticus gut and identified as Bacillus paralicheniformis through phenotypic analysis and 16S rRNA gene sequencing. These probiotic isolates demonstrated strong antagonistic activity against the pathogenic A. veronii , ecto-enzymatic producing activities, and high bile salt tolerance. After comprehensive screening, seven Bacillus strains met safety criteria, exhibiting high sensitivity to antimicrobials, non-hemolytic activity, and non-pathogenic in the challenge test. This study confirms A. veronii as the causative agent of tilapia summer mortality and presents promising autochthonous probiotics for preventive strategies in aquaculture.