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

Sarisha Singh,Roshini Govinden,Prabashni Lekha,Kugenthiren Permaul,Bruce Sithole

doi:10.1186/s40643-021-00361-x

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.

Highlights

An estimated 240 million tons of plastics are produced globally every year
This study reports the practicality of employing Bacillus thuringiensis in a cyclic fed-batch fermentation (CFBF) cultivation using glucose-rich hydrolyzate derived from enzymatic saccharification of pulp and paper mill sludge as the sole carbon source
This study assessed the ability and efficiency of B. thuringiensis to assimilate glucose-rich hydrolyzate produced by enzymatic saccharification of paper mill sludge (PPMS)

Summary

Introduction

An estimated 240 million tons of plastics are produced globally every year. Most of these plastic materials are petroleum-based and are resistant toSingh et al Bioresour. An estimated 240 million tons of plastics are produced globally every year. Most of these plastic materials are petroleum-based and are resistant to. Research into PHAs has intensified since the discovery that PHA-bioplastics are non-toxic, biodegradable and that their mechanical, physical, and thermal characteristics are similar to fossil fuel-based plastics, such as polypropylene and polyethylene (Muhammadi et al 2015). PHAs have the potential to replace fossil fuel-derived plastics. The PHA production cost is a significant problem as carbon source alone accounts for 45% of the total production cost, hindering the market growth of PHA-based plastics (Annamalai et al 2017). For PHAs to be cost-competitive with petroleum-derived plastics, sustainable, renewable, and cheaper alternative carbon source substrates need to be explored. Employing microbial strains capable of high cell density growth and PHA production in waste-derived cultivation media will be beneficial

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Conclusion