Isolation and selection of lactic acid bacteria from Thai indigenous fermented foods for use as probiotics in tilapia fish Oreochromis niloticus

In this study, 119 bacterial strains were isolated from various samples such as healthy tilapia fish (Oreochromis niloticus), water and sediment around the culture fish-cages, and several kinds of traditional fermented foods. These bacterial isolates were screened for antibacterial activities against bacterial fish pathogens i.e. Aeromonas hydrophila, A. caviae and Streptococcus agalactiae using an agar well diffusion assay. The isolate CR1T5 derived from fermented rice showed the highest antibacterial activity against all three fish pathogens tested. It was identified as Lactobacillus plantarum by using both conventional and molecular methods. The other probiotic properties were evaluated in vitro which revealed that strain CR1T5 tolerated the simulated gastrointestinal conditions well, showed high capacity to adhere intestinal mucosa and did not lyse red blood cells. The efficiency of L. plantarum CR1T5 was also examined in vivo. O. niloticus were employed in the feed-trial experiments. Fish fed a diet containing strain CR1T5 (10⁸ CFU g^-1 feed) displayed not only no mortality but also growth improvement. At the end of feed-trial, fish were challenged by intramuscularly injection of A. hydrophila (3.1x10⁵ CFU) The L. plantarum CR1T5-fed fish survived (87.5%) better than the fish fed a control diet (12.5%) after a two week-challenge. This study clearly shows that L. plantarum strain CR1T5 is a promising probiotic candidate for farmed fish.