Optimization of biomass-producing conditions of Micrococcus sp S-11 for L-cysteine production

Micrococcus S-11 isolated from sediments could transform racemic 2-Amino-Delta(2)-thiazoline-4-carboxylic acid (ATC) into L-cysteine. The optimal carbon and nitrogen source for its biomass production were glucose and urea. The optimal culture conditions for biomass production were investigated through statistical experiment design and data analysis. A screening test was first conducted on ten process variables using a Plackett-Burman design, from which three parameters including glucose, urea and rotational speed were chosen as significant ones influencing biomass production. Then these three variables were optimized by Box-behnken experimental design and response surface methodology, and a multinomial equation was constructed to describe the correlation between the biomass production and the three tested variables. By solving to this equation, the predicted maximum biomass was obtained at 11.30 g/L when the culture conditions were glucose 21.98 g/L, urea 4.75 g/L and rotational speed 124 rpm. The validation experiments were carried out under the optimal conditions, from which the average biomass obtained was 11.26 g/L close to the predicted biomass (11.30 g/L), which was 80.7% higher than the one 6.23 g/L obtained under the initial conditions. The results from validation experiments verified the accuracy of the model in terms of depicting the biomass production of Micrococcus sp. S-11.