In-vitro and in-vivo assessment of the bactericidal potential of peracetic acid and hydrogen peroxide disinfectants against A. hydrophila infection in Nile tilapia and their effect on water quality indices and fish stress biomarkers

This study aimed to assess the in vitro and in vivo disinfectant potential of peracetic acid (PAA) (1 mg/L) and hydrogen peroxide (H₂O₂) (20 mg/L) on the physicochemical and microbiological water quality parameters of fish aquaria, the microbial density of Nile tilapia muscular tissue, fish hepatic cortisol levels, and antioxidant biomarkers. In vitro, PAA and H₂O₂ reduced A. hydrophila colony viability by 5 log units after 30 and 5 min of contact time, respectively. PAA and H₂O₂ were added to aquaria water twice a week for the three-week experiment. Increased fish escape reflexes were observed only in the PAA group, which returned to normal within 10 min. No mortalities were reported in either the PAA or H₂O₂ groups. An in vivo experimental challenge with a pathogenic strain of A. hydrophila revealed a 20% reduction in mortality in the PAA group, with no mortalities in the H₂O₂ group. Cortisol levels and antioxidant markers were measured to assess the impact of PAA and H₂O₂ on fish health. Cortisol levels in the PAA and H₂O₂ groups were significantly higher than in the control group after disinfectant exposure, but they progressively returned to normal. A significant reduction in superoxide dismutase (SOD) and catalase (CAT) activity, along with considerably higher glutathione peroxidase (GPx) and malondialdehyde (MDA) enzymatic activity, was observed in the PAA and H₂O₂ groups compared to the control group. A substantial increase in total antioxidant capacity (TAC) was recorded in the PAA group. Physicochemical analyses revealed reduced pH and increased dissolved oxygen levels in the PAA and H₂O₂ groups. Microbiological analyses showed a significant reduction in bacterial density in water by 64% and 76% after 30 min of exposure to PAA and H₂O₂, respectively, with a non-significant increase in microbial count after bacterial challenge. Additionally, aerobic bacterial count, Aeromonas spp., and psychotropic bacterial count in fish muscle showed a significant reduction in the H₂O₂ group compared to the PAA and control groups before and after infection. The study concludes that regular application of PAA and H₂O₂ can temporarily reduce bacterial load in aquaria and fish muscle, regulate stress responses, and improve fish health by reducing A. hydrophila-induced infections and improving survival.