The effect of the concentration of yeast extract on glucose isomerase production. Yeast extract provides nitrogen for production of glucose isomerase but high concentration will inhibit the bacterial growth. In this study, we determined the optimal concentration of yeast extract is 1%. Addition of yeast extract (0–1%) in induction medium resulted in an increase of enzyme activity, but yeast extract also exhibited the inhibitory effect in high level (1.25%). a, b, c, and d indicate the significant difference between groups.