Comparative evaluation of Lacticaseibacillus paracasei and Lacticaseibacillus rhamnosus from traditional fermented foods on boosting fish growth and disease resistance

The growing demand for sustainable aquaculture has promoted the use of probiotics and synbiotics as alternatives to antibiotics to enhance fish growth, immunity, and disease resistance. Lactic acid bacteria (LAB) isolated from traditional fermented foods are promising candidates to develop aquafeeds. This study aimed to isolate and characterise probiotic LAB strains from fermented foods and evaluate their synbiotic potential with Gracilaria for the health and performance of Oreochromis niloticus (Nile tilapia). Lacticaseibacillus paracasei from mishti doi and Lacticaseibacillus rhamnosus from kombucha, were identified by 16S rRNA gene sequencing. Probiotic properties such as pH, NaCl (up to 6%), pepsin and trypsin tolerance, auto-aggregation, hydrophobicity, lactic acid production and antimicrobial activity against Vibrio harveyi were determined. The prebiotic effect of Gracilaria was evaluated in vitro followed by a five-week feeding trial in Nile tilapia. The strains were Gram-positive, catalase-negative, non-motile and tolerant to simulated gastrointestinal conditions. Auto-aggregation ranged from 27.41% to 64.70%, and hydrophobicity was 53.6%. L. paracasei produced more lactic acid (1.20 ± 0.068 mg/mL) than L. rhamnosus (0.88 ± 0.265 mg/mL). However, L. rhamnosus showed better antagonistic activity against V. harveyi. HPLC analysis indicated lactic acid as the major organic acid. Gracilaria significantly increased LAB growth in vitro (p < 0.05). The synbiotic diets significantly improved fish performance in the feeding trial, and the mixed-strain synbiotic group had the highest weight gain (p < 0.001) and catalase activity (49.67 ± 1.53 U/mL) after challenge with V. harveyi. These findings demonstrate the potential of combining L. paracasei, L. rhamnosus and Gracilaria, as sustainable synbiotic feed additives for tilapia aquaculture.