It was also found that the production of c-PGA was almost eliminated when the above isolated ten strains were cultivated in the control medium which contains all the components in the selective medium except the addition of 30 g/L CaCO3. Even with addition of CaCO3, the productivity was still low (less than 4.0 g/L), thus further optimization of medium composition was necessary to improve the c-PGA production with the strain B. subtilis C10.