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

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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.