Screening And Optimizing Fermentation Conditions For Polyhydroxyalkanoates-producing By Microbial Mixed Culture

Polyhydroxyalkanoates (PHA) which was fermented by a variety of bacteria is a kind ofbiodegradable synthetic aliphatic polyester. This research is of great significance to protectenvironment and make sustainable use of resources. In this study, we screened two strainswhich could synthetize PHA, optimized the fermentation process by the response surfacemethod and characterized the chemical and physical properties of fermented product.(1) Two bacteria strains were isolated from the natural environment in Xinjiang, whichwas capable of synthesizing PHA. The bacteria strains were identified as Burkholderiacepacia WN-H41and Bacillus sp.1175via physiological and biochemical tests as well as16S rDNA sequence analysis. The bacteria strains can symbiosis in agar plate.(2) The two strains of Burkholderia cepacia WN-H41and Bacillus sp.1175were object ofthis study, which were used to mixed culture to product PHA and optimize the fermentationprocess. The influence of different inoculating age, inoculation proportion, fermentationperiod, carbon source, nitrogen source et al on the fermentation was investigated by the singlefactor test. Plackett-Burman design was used to evaluate the influence of related factorsincluding peptone, MgSO4·7H2O and soluble starch, and results showed that three factorshave played an important role on the yield of PHA. The steepest ascent path and theBox-Behnken design were used to further optimization, and the results of optimization:peptone17g/L, MgSO4·7H2O0.4g/L, soluble starch39g/L. Opon these conditions, the yieldof PHA was obtained to7.668g/L, and had a high coincidence with predictive value, whichhad increased2.15-ploid compared to the maximum value acquired in the single factor tests.(3) The structure, molecular weight, thermal and tensile properties of and mixedbacterium fermentation production were analyzed, and the results were compared with thesingle bacterium fermentation production. The results showed that three kinds of polymerswere PHB. In addition, compared with single bacterium fermentation, PHB of mixedfermentation had a higher average molecular weight (Mn＝11.2×104) and narrowermolecular weight distribution (PDI＝1.16). Thermal properties of PHB had no significantdifference among the three types of fermentation productions. Besides, there was a strongpositive correlation between the tensile strength and molecular weight of PHB.(4) We enhanced production of PHB with the strain of B. cepacia WN-H41by physicaland chemical mutation. A mutant strain designated as EMS-40-3was successfully isolated. Itwas increased about54.75%in comparison with that of the original strain. The subcultureexperiments indicated that the high producer was stable.