A 7-week feeding trial was conducted to evaluate the combined effects of propionic acid (PA, 5 or 10 g/kg) and a multi-strain Bacillus spp. (Bacillus subtilis IS02 (accession no. JN856456) and B. licheniformis IBRC-M 11,319) (1.7 × 107 CFU/g) probiotic in a plant protein source (PP)–rich diet (∼70% of dietary protein derived from PP sources) on performance of Asian sea bass (Lates calcarifer) fry (initial body weight 2.97 ± 0.11 g). In this regard, six isoproteic (∼48%) diets were formulated as follows: a control (without supplementation of the additives); probiotic (only contained Bacillus spp. mixture); 5 g PA/kg diet; 10 g PA/kg diet; probiotic + 5 g PA/kg diet, and probiotic + 10 g PA/kg diet. Specific growth rate in fish fed with 10 g PA/kg (2.84 ± 0.1%) and diets contained blends of probiotic and PA (2.76 ± 0.19% in probiotic + 5 g PA, and 2.79 ± 0.04% in probiotic + 10 g PA) was better than in the control (2.45 ± 0.1%) (P < 0.05). Feed conversion ratio in fish fed with 10 g PA/kg (0.92 ± 0.12) and diets contained blends of probiotic and PA (0.94 ± 0.06 in probiotic + 5 g PA and 0.91 ± 0.02 in probiotic + 10 g PA) was better than in the control (1.24 ± 0.08) (P < 0.05). Digestive enzymes including α-amylase, total alkaline proteases, and bile salt dependent lipase activities improved in fish fed diets contained additives. The activity of glutathione-S-transferase and glutathione reductase enhanced in the liver of fish fed diets contained additives. The relative abundance of lysozyme, interleukin 1β, and insulin-like growth factor-1 genes mRNA transcript showed multifold increase in the liver of fish fed with the 10 g PA/kg and diets contained blends of probiotic and PA (P < 0.05). By considering the above mentioned results, supplementing a PP-rich diet with 10 g PA/kg diet or combination of PA and a mixture of Bacillus spp. probiotic recommended for L. calcarifer.
