Abstract: The fermentation medium for glutamate decarboxylase (GAD) production by Enterococcus faecium was optimized by bidirectional one-factor-at-a-time and Taguchi methods. The results indicated that the peptone-sucrose-beef extract (PSB) medium was found to be the most suitable for GAD production among four tested media. However, CaCl2, K2HPO4 and ammonium citrate in the PSB medium were unfavourable for GAD production. Peptone, sucrose, sodium acetate and monosodium glutamate (MSG) had significant effects on GAD production (P < 0.01), which was not significantly affected by beef extract (P > 0.10). The optimal culture medium predicted by Minitab software consisted of 15 g/L peptone, 10 g/L beef extract, 12.5 g/L sucrose, 6.0 g/L sodium acetate, 10 g/L MSG, and 1.0 g/L Tween 80. The predicted total GAD activity obtained by using the optimized medium was (399.30 ± 12.51) U, which was not significantly different from the experimental value ((384.26 ± 10.32) U). The constituents of the modified PSB medium were much fewer compared with the initial one. However, the modified PSB medium was more suitable for GAD production, in which the total GAD activity produced by E. faecium was improved by 45.71% as compared to the initial one.

Key words: Enterococcus faecium, glutamate decarboxylase, medium, optimization, bidirectional one-factor-at-a-time method, Taguchi method