Application of response surface methodology to optimize production of poly-ɛ-lysine by Streptomyces albulus IFO 14147

Abstract Response surface methodology (RSM) were employed to study the effect of yeast extract, glucose, ammonium sulfate, and initial medium pH on the production of poly-ɛ-lysine (ɛ-PL) by Streptomyces albulus IFO 14147 in shaken flask cultures. The results of first-order factorial design experiments showed that the linear terms of yeast extract, glucose, and ammonium sulfate had significant effect on ɛ-PL production, but the initial medium pH at the tested range exhibited insignificant effect on ɛ-PL production. It was predicted that decreasing the concentration of yeast extract, glucose, and ammonium sulfate enhance ɛ-PL production. Furthermore, the results of the four-factor-two-level (2 4 ) factorial experiments indicated that the first order with interactions model ( R 2 value of 0.9660) appeared to be more adequate than the pure first-order model ( R 2 value of 0.8759). The optimum composition was then assessed by using a central composite design (CCD). The experimental results of CCD were fitted with a second-order polynomial equation by a multiple regression analysis. The determination coefficient R 2 was 0.816, the Fisher F -test was significant at upper 5% level and the lack of fit was insignificant at 5% level. All of these indicated a good adequacy of the second-order polynomial model proposed to explain the data observed. The optimal ɛ-PL yield (8.90 ± 2.14 g/L) determined by the CCD was predicted at the regions where concentrations of yeast extract, glucose, and ammonium sulfate were at 2.86, 23.82, and 0.14 g/L, respectively. Verification of the calculated optimum was done with a culture medium representing this optimal point and yielding 8.13 ± 1.87 g/L (average of three repeats). The yield of ɛ-PL by S. albulus IFO 14147 was increased significantly by 984%, from 0.75 to 8.13 g/L when the strain was cultivated in the optimal medium developed by surface response methodology, as compared to conventional medium M3G used in the literature. The ɛ-PL thus produced was well characterized by chemical tests, amino acid analysis and gel filtration technique.