With the aim to enhance the efficiency of utilization of crystalline methionine supplemented in the high plant-protein diet for grouper (Epinephelus coioides), this study used Bacillus subtitlis spore as a probiotic additive in the diet to shorten the absorption time difference between protein-bound amino acid and crystalline methionine. The study was conducted in two parts. In the first part, juvenile groupers were fed for 14 weeks with 5 experimental diets: fish meal diet, high plant-protein diet with/without crystalline methionine, as well as with/without B. subtitlis spore separately. Growth performance, PER, protein digestibility, amino acid digestibility, non-specific immune responses, and free amino acid concentration in both muscle and serum were assessed. The second part was a time-series study on serum free amino acids concentration after a force-feeding experiment. The results showed that crystalline methionine supplementation in the high plant-protein diet effectively improved the growth of E. coioides (P<0.05). However, B. subtitlis spore supplementation did not affect fish growth performance significantly (P>0.05). A delay in the appearance of peak serum amino acid concentration was observed when fishmeal was partially replaced by soy protein. On the other hand, the force-feeding experiment showed that serum essential amino acid (include methionine) concentrations droped drastically after they reached the peak concentrations from being forced-fed with the B. subtilis containing diet. Supplementation of crystalline methionine seemed to ease the drop of serum methionine concentration. Based on these results, it is concluded that addition of B. subtitlis spore in high plant-protein diet for the grouper does not enhance the utilization of crystalline methionine, but supplementation of crystalline methionine significantly improve the growth performance of the grouper.

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