Ameliorative effects of Lactobacillus plantarum L-137 on Nile tilapia (Oreochromis niloticus) exposed to deltamethrin toxicity in rearing water

•Deltamethrin (DLM)-treated fish showed increased creatinine, urea and bilirubin as well as hepatic enzymes (ALP, AST, and ALT).•Blood haemato-immunological variables were decreased in fish exposed to DLM.•DLM toxicity downregulated the transcription of IL-1β and IL-8, while upregulated HSP70 and CASP3.•The histopathological images revealed damage in gills, intestine, spleen, and liver of fish exposed to DLM.•Lactobacillus plantarum L-137 presented protective effects and restored the antioxidative capacity and the wellbeing of Nile tilapia. Deltamethrin (DLM) is a synthetic pyrethroid used for agricultural purposes to control insects and has been found to pollute the aquatic environment and leads to serious health problems. Lactobacillus plantaruml-137 (L-137) has gained more popularity as functional supplement for its immunomodulatory effects and antioxidant potential. This study was designed to examine the potential of l-137 on liver function, histopathology, immune and antioxidant related gene expressions in Nile tilapia exposed to subacute DLM for 30 days. Fish (mean weight of 28.18 ± 1.34 g) was distributed into four groups (triplicates): the first and second groups fed the control diet, while the third and fourth groups fed l-137 at 50 mg/kg and the second and fourth groups were exposed to DLM (15 μg/L) in rearing water (control, DLM, l-137 and DLM + L-137, respectively). DLM-treated fish groups showed a significant increase in blood biochemical parameters (creatinine, urea and bilirubin) as well as hepatic enzymes (ALP, AST and ALT) (P