| Summary: | In modern aquaculture, dietary supplementation of probiotics is a novel approach for enhancing growth performance of slow-growing fish. However, the actual role of probiotics in regulating muscle growth at cellular and molecular levels in fish still needs to be clarified. In this study, we hypothesized that host gut derived probiotic bacteria would enhance cellular muscle growth, and upregulate growth-related gene expression in slow-growing Malaysian mahseer Tor tambroides. Therefore, three host-associated probiotics (Bacillus sp. AHG22, Alcaligenes sp. AFG22, and Shewanella sp. AFG21) were isolated from the gastro-intestinal tract of T. tambroides and screened based on their digestive enzyme activity. A fishmeal and casein based control diet (40% crude protein and 10% lipid) was formulated, and three different probiotic supplemented diets were prepared by immersing the control diet in each isolated host-derived bacteria, suspended in sterile phosphate buffered saline (PBS), to achieve a final concentration of approximately 1.0 × 108 CFU g−1 feed. Triplicate groups of T. tambroides juveniles (initial weight 1.39 ± 0.06 g) were stocked in twelve glass aquaria (100 L capacity) with stocking density of 20 individuals per aquarium. The feed was applied twice daily at 3.0% of the fish body weight per day for 90 days. Growth performance (weight gain and specific growth rate) of T. tambroides juveniles were significantly higher in Alcaligenes sp. AFG22 and Bacillus sp. AHG22 supplemented diet treatments. Muscle morphometric analysis revealed that dietary supplementation of host-associated probiotic bacteria did not influence the frequency distribution of hyperplastic (class 10) small diameter fibers (≤10 μm). However, hypertrophic (Class 50, Class 60 and Class 70) large diameter fibers (>50 μm) were significantly higher in Alcaligenes sp. AFG22 and Bacillus sp. AHG22 supplemented treatments, indicating that increased growth rate of T. tambroides in these treatments was mostly governed by increased muscle fibers hypertrophy, rather than by hyperplasia. Real-time PCR data demonstrated that the relative mRNA expression of GH and IGF1 was upregulated in juvenile T. tambroides fed the diets supplemented with Alcaligenes sp. AFG22 and Bacillus sp. AHG22. These results indicate that host-derived probiotics, especially Alcaligenes sp. AFG22 and Bacillus sp. AHG22, have a significant potential as autochthonous probiotics for the stimulation of growth performance of slow-growing T. tambroides in aquaculture.
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