Optimization of cultivation medium and cyclic fed-batch fermentation strategy for enhanced polyhydroxyalkanoate production by Bacillus thuringiensis using a glucose-rich hydrolyzate

Abstract

The accumulation of petrochemical plastic waste is detrimental to the environment. Polyhydroxyalkanoates (PHAs) are bacterial-derived polymers utilized for the production of bioplastics. PHA-plastics exhibit mechanical and thermal properties similar to conventional plastics. However, high production cost and obtaining high PHA yield and productivity impedes the widespread use of bioplastics. This study demonstrates the concept of cyclic fed-batch fermentation (CFBF) for enhanced PHA productivity by Bacillus thuringiensis using a glucose-rich hydrolyzate as the sole carbon source. The statistically optimized fermentation conditions used to obtain high cell density biomass (OD600 of 2.4175) were: 8.77 g L−1 yeast extract; 66.63% hydrolyzate (v/v); a fermentation pH of 7.18; and an incubation time of 27.22 h. The CFBF comprised three cycles of 29 h, 52 h, and 65 h, respectively. After the third cyclic event, cell biomass of 20.99 g L−1, PHA concentration of 14.28 g L−1, PHA yield of 68.03%, and PHA productivity of 0.219 g L−1 h−1 was achieved. This cyclic strategy yielded an almost threefold increase in biomass concentration and a fourfold increase in PHA concentration compared with batch fermentation. FTIR spectra of the extracted PHAs display prominent peaks at the wavelengths unique to PHAs. A copolymer was elucidated after the first cyclic event, whereas, after cycles CFBF 2–4, a terpolymer was noted. The PHAs obtained after CFBF cycle 3 have a slightly higher thermal stability compared with commercial PHB. The cyclic events decreased the melting temperature and degree of crystallinity of the PHAs. The approach used in this study demonstrates the possibility of coupling fermentation strategies with hydrolyzate derived from lignocellulosic waste as an alternative feedstock to obtain high cell density biomass and enhanced PHA productivity.

Development and Characterization of Electrospun Biopapers of Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) Derived from Cheese Whey with Varying 3-Hydroxyvalerate Contents

“…The lower thermal stability as well as differences in the thermal stability within the CW-derived PHBV copolyesters can be ascribed to the presence of residual cations or impurities from the biological source used in the production process of the microbial copolyester . These values are, however, in agreement with those reported by other previous studies that showed that the thermal decomposition of the microbial copolyester occurred in a single and sharp degradation step from approximately 270 to 280 °C. , One can also see that a low-intense shoulder was formed at approximately 350 °C, which is due to residual carbonaceous material with higher thermal stability that has also been observed in some previous studies. , This mass loss may be related to the thermal degradation of proteins or other impurities as well as degradation products such as crotonic acid (CA) and oligomers with new crotonyl chain ends . In this regard, it has been reported that thermal degradation of PHBV is caused by chain scission, via random process, and hydrolysis, resulting in a reduction of the biopolymer’s M W and formation of CA. , Finally, the amount of residual mass was very similar for all the CW-derived PHBV copolyesters, between 0.2 and 0.7%.…”

“… 66 , 67 One can also see that a low-intense shoulder was formed at approximately 350 °C, which is due to residual carbonaceous material with higher thermal stability that has also been observed in some previous studies. 68 , 69 This mass loss may be related to the thermal degradation of proteins or other impurities as well as degradation products such as crotonic acid (CA) and oligomers with new crotonyl chain ends. 70 In this regard, it has been reported that thermal degradation of PHBV is caused by chain scission, via random process, and hydrolysis, resulting in a reduction of the biopolymer’s M W and formation of CA.…”

Polyhydroxyalkanoate production from rice straw hydrolysate obtained by alkaline pretreatment and enzymatic hydrolysis using Bacillus strains isolated from decomposing straw

“…Moderate PHA yield of 0.15 g/g was obtained with respect to the sugar consumed in the B. cereus isolates (Table 5), which was equivalent to 59.5 g PHA per kg rice straw in case of B. cereus VK92, considering also the losses during the pretreatment. It is possible to further improve the PHA content and productivity in the isolates by medium optimization, e.g., with respect to carbon:nitrogen ratio, phosphate level, aeration, etc., and by using fed-batch mode of cultivation under controlled conditions (Singh et al 2021). Moreover, strategies to further improve the resource-and cost-efficiency need to…”

Bacterial Isolates from Decomposing Rice Straw used for Polyhydroxyalkanoate Production from Straw Hydrolysate Obtained by Alkaline Pretreatment and Enzymatic Hydrolysis

“…Further improvements in PHA content and productivity are possible by medium optimization e.g. with respect to carbon:nitrogen ratio and phosphate level, and using fed-batch mode of cultivation under controlled conditions (Singh et al 2021).…”