Adaptation of terrilactibacillus laevilacticus SK5-6 for growth and d-lactic acid production in a low yeast extract concentration medium

Abstract

Lactic acid has been extensively used in food, personal care, and pharmaceutical products. Besides, lactic acid can be used as the sole feedstock in the synthesis of polylactic acid (PLA), the biodegradable plastic. The demand of the bioplastics including PLA has been increased due to the environmental impact of petroleum based plastic; therefore, this expedited the market demand of lactic acid. It should be noted that the polymer grade lactic acid is unlike those commercially available products for food and pharmaceutical applications. The high purity of stereoisomer either D-lactate or L-lactate is mandatory in PLA polymerization. In addition, the production cost should be low in order to be competitive with the plastic commodities widely used nowadays. To fill those production gaps, one needs to develop the simple yet effective fermentation platform for lactic acid production. Microbial strain is one of the crucial factors. Terrilactibacillus laevilacticus SK5-6, a novel lactic acid producing genus, was preliminary isolated and screened for the production of D-lactic acid at a remarkable high production rate in a small scale culture using the enriched culture medium containing yeast extract. In industrial lactate fermentation, yeast extract is considered as the key component that facilitate cell growth and lactate production yet mainly contribute to high production cost. In this thesis, phenotypic adaptation technique was implemented in T. laevilacticus SK5-6 to produce D-lactic acid in the production medium containing a lowering concentration of yeast extract. With the new adaptive isolate of T. laevilacticus SK5-6 to be obtained, a lower medium cost required in lactic acid production was acquired. During phenotypic adaptation, it was found that at the beginning of the preculture transfer (T4-T8), the low final cell biomass concentration was obtained. After that, the final cell biomass concentration was increased and became comparable to the theoretical value (5% biomass conversion yield from glucose). Later, the adapted isolate T. laevilacticus SK5-6 T20 was used in D-lactic acid fermentation. The similar final product concentration and yield with the prolonged production to that obtained in the fermentation by the wild type isolate in the original base case medium were obtained. From the findings in this thesis, phenotypic adaptation was claimed as an excellent tool in strain improvement for enhancing the fermentation process efficiency.