Probiotics have increasingly gained interest as alternatives to antibiotics in controlling infectious diseases in aquaculture. This research aimed to isolate and evaluate lactic acid bacteria (LAB) for potential use as probiotics for Pangasius bocourti. A total of 656 LAB isolates were obtained from fish gut and fermented beef samples, of which 367 isolates displayed antagonistic activity against Aeromonas hydrophila FW52 and/or Streptococcus agalactiae F3S which were employed as indicator fish pathogens. Among these antagonistic LAB, only 18 isolates produced bacteriocin-like activity. These were further evaluated in vitro for other probiotic properties. After in vitro screening, the isolate L34b-2 obtained from Thai indigenous fermented beef was chosen as a suitable probiotic LAB for in vivo studies based on its remarkable probiotic characteristics. These included broad-spectrum bacteriocin-like activity, tolerance to simulated gastrointestinal tract conditions, mucin adhesion ability, co-aggregation ability with fish pathogens tested, non-blood hemolysis, antibiotic susceptibility and protease enzyme production. Identification of the isolate L34b-2 using conventional methods together with 16S rDNA analysis revealed that it belonged to Lactobacillus paraplantarum. Then, the effects of 60-day-dietary administration of L. paraplantarum L34b-2 on fish growth, innate immune responses and disease resistance were investigated in Pangasius bocourti. In feed-trial studies, after feeding for both 30 and 60 days it was found that the Pangasius fish fed with basal diet containing 107 CFU g−1 L. paraplantarum L34b-2 had significantly (p < 0.05) higher weight gain and specific growth rate, and the lowest feed conversion ratio, compared to the control fish that received only basal diet. For immune response studies, it was found that the fish fed with the probiotic strain L34b-2, when compared to the control fish, had significantly higher lysozyme, alternative complement and bactericidal activities but no significant differences (p > 0.05) in phagocytic and respiratory burst activities. After 60-day feeding, fish were subjected to challenge tests by intraperitoneal injection with 106 CFU of a virulent Aeromonas hydrophila FW52 and rearing was continued for two more weeks. As expected, fish that received L34b-2 had a significantly higher (p < 0.05) post challenge survival rate than the control group. These results clearly show that L. paraplantarum L34b-2 displayed beneficial effects on P. bocourti as a probiotic by which it not only promoted growth but also enhanced innate immunity and disease resistance against A. hydrophila.
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