Studies On Production Of Monomers Of Biodegradable Polymers By Lactobacilli

Lactic acid bacteria (LAB) is a group of gram-positive bacteria that could produce a large amount of lactic acid from available carbohydrates. They are catalase-negative without endospores and movement. According to the metabolic types of sugar, LAB was classified as three categories: homofermentation, heterofermentation and facultative heterofermentation. Nowadays LAB is widely used in the fields of food, health food, medicine, ensilage, scientific research and so on. The product of industry materials, especially the monomer of new patterns of polyester, is paid on more and more close attention.1,3-propanediol (1,3-PD) is the monomer of polytrimethylene terephthalate (PTT). Because of its biodegradability and high-performance, PTT has come to be a hotspot during the exploitations of new polyester over the world. In recent years, publications were focus on the production of 1,3-PD by Klebsiella pneumonide and clostridium in both China and abroad while the studies about production of PTT by LAB were not enough as other bacteria. Yet the overproduction of 1,3-PD of LAB by metabolism engineering has potential possibility.In this paper, a strain of lactobacilli isolated from spoilage wine, which could produce 1,3-PD by glucose-glycerol cofermentation, was identified as Lactobacillus brevis basing on the results of biochemical experiments and PstI digestion of PCR production of 16S rRNA, and was named as Lb. brevis W5-17. To analyze the enzyme activities of glycerol dehydratase (GDHt) and 1,3-propanediol NAD+ oxidoreductase (PDOR), which were the two key enzymes in production of 1,3-PD by Lb. brevis W5-17, the conditions of ultrasonic disruption cell were made attempt. The result showed that, cells of 20 mL fermentation broth were resuspended in 20 mL potassium phosphate buffer (pH7.0) after centrifugation, 7/6 s ultrasounds on, 4 s ultrasounds off, 99 times, 300 w power. Lb. brevis W5-17 could produce reducing power NADH in the metabolism of glucose and NADH could reduce glycerol to 1,3-PD under anaerobic conditions. When the ratio of glucose to glycerol was 0.05 mol/L : 0.1 mol/L, the yield of 1,3-PD was 11.20 mmol/L and the conversion of glycerol to 1,3-PD was 0.30 mol/mol by Lb. brevis W5-17 for 70 hours' incubation at 31℃. The highest enzyme activities of GDHt and PDOR were 0.84 U/mL and 3.80 U/mL, respectively. Moreover, it was firstly found that Lb. brevis W5-17 could also produce 1,3-PD under aerobic conditions though the production was only 7.12 mmol/L and the enzyme activities were lower than those under anaerobic conditions. This study was attributing a foundation for production of 1,3-PD by LAB.LAB could produce L-lactic acid as the monomer of another biodegradable polyester, poly lactic acid (PLA). The second parts of this thesis worked on the fermentation of L-lactic acid from corn steep liquor in point view of 'Biorefinery' in order to realize the high added value production of scrap biomass and more, to keep the carbon ecological balance.Corn steep liquor that obtained from corn steep hydrolyzed by HCl contained 72-78 g/L of D-xylose, 10-12 g/L of D-glucose and 9-11 g/L of L-arabinose analyzed by HPLC. Lactobacillus plantarum S34 screened from the strains stocks in our laboratory could convert most monosaccharides in the corn steep liquor to L-lactic acid. The research on metabolic characters of Lactobacillus plantarum S34 showed that the production of L-lactic acid and acetic acid were 9.0 g/L and 2.9 g/L after 50 h' incubation from the corn steep liquor (1:4) medium, respectively. And the percentages of the consumed glucose, xylose, and arabinose in weights were 100%, 29.4% and 100%, respectively. For the advancement of the utilization of xylose, the optimized corn steep liquor medium was as following (g/L): corn steep liquor (1 : 4), bean peptone 20, (NH₄)₂C₆H₅O₇ 2 , K₂HPO₄ 2 , CaCO₃ 10 , pH6.0. By the end of fermentation for 50 h in 5 L fermentor, the production of L-lactic acid and acetic acid were up to 32.0 g/L and 14.6 g/L, respectively. The percentages of the consumed xylose in weights went up to 29.4% from 39.0 % while glucose and arabinose were almost consumed out.