Optimization of culture conditions for violacein production by a new strain of Duganella sp. B2

Duganella sp. B2 isolated from Xingjiang Municipality of China was found to be a novel violacein-producing bacterium. In this study, culture variables affecting violacein production were optimized statistically in shake flask experiments. The medium pH, culture volume, and medium components of potassium nitrate, l-tryptophan, and beef extract were identified by two-level Plackett–Burman design as the most significant factors for violacein production. The path of steepest ascent was undertaken to determine the optimal region of five significant factors. Box–Behnken design and response surface analysis were adopted to further investigate the interaction between the variables and determine the optimal values for violacein production. The optimal culture conditions for violacein production were: pH 6.71, potassium nitrate 1.18 g/L, ammonium ferrous sulfate 0.08 g/L, dipotassium hydrogen phosphate 0.25 g/L, magnesium sulfate 0.75 g/L, beef extract 1.53 g/L, l-tryptophan 0.74 g/L, soluble starch 13.0 g/L, 25 mL medium in a 250 mL flask, with an inoculum size of 10% (v/v). 1.62 g/L crude violacein was achieved using the optimized conditions after 32 h of cultivation. This proved to be much higher than any other violacein-producing strains so far reported.