The effect of the concentration of D-xylose on glucose isomerase production. We found that the increase of concentration of D-xylose (1–1.5%) will result into improvement of bacterial growth and activation of glucose isomerase biosynthesis. However, the higher concentration of D-xylose (2–2.5%) could inhibit the bacterial growth, which decreased enzyme activity. Statistical analysis revealed no significant difference between 1% and 1.5% D-xylose on enzyme production; therefore, we chose 1% of D-xylose for further optimization experiments. a and b indicate the significant difference between groups.