Response surface and contour plots of c-PGA production by Bacillus subtilis RKY3 showing mutual interactions between (a) glycerol and glutamic acid, (b) yeast extract
and glutamic acid, (c) K2HPO4 and glutamic acid, (d) yeast extract and glycerol, (e) K2HPO4 and glycerol, and (f) K2HPO4 and yeast extract.