Screening of Weissella bacterial strains for Production of Bioactive Metabolites on Different Substrates

Sammanfattning

Lactic acid bacteria (LAB) have been recognized for their potential to hydrolyse different types of substrates, including plant-based proteins, releasing products of nutritional and biotechnological interest. This thesis investigated three Weissella strains, including AYR-17, AYR-1, and AYO-14, for their ability to grow on different substrates, break down proteins and produce bioactive metabolites. Bacterial growth was assessed on different substrates, including a protein-rich medium supplemented with glucose, a starch-rich potato medium, and MRS broth, which served as control. HPLC analysis revealed different organic acid profiles that varied with substrate and oxygen availability, consistent with the heterofermentative metabolism of these strains. Lactate was the dominant organic acid on MRS, reaching ~7.50-8.50 g/L under both conditions, whereas on the protein-glucose medium lactate was lower (approximately 2.60-5.0 g/L), and on potato it remained stable at ~0.5 g/L throughout cultivation. Aerobic cultivation favoured acetate production on both MRS (~6.0-8.0 g/L) and protein-glucose (~4.0-6.0 g/L), while anaerobic cultivation on MRS resulted in the higher concentration of ethanol (3.0-3.6 g/L). On potato, acetate remained at a constant level at approximately 4.0-4.3 g/L, suggesting carryover from the inoculum rather than production occurring. HPAEC-PAD analysis confirmed the strain’s ability to break down starch and synthesize glucose-rich heteropolysaccharides containing minor amounts of arabinose and galactose. Strain AYO-14 (anaerobic) exhibited growth-dependent EPS production, yielding 90.23 µg/mL, while strain AYR-17 (aerobic) exhibited growth-independent EPS production, with a yield of 157.04 µg/mL. Although o phthaldialdehyde (OPA) analysis indicated a net decrease in free amino groups during cultivation, further analyses are needed to determine the strains' ability to digest yellow pea protein. Overall, these findings suggest that when the strains utilise monosaccharide substrates, carbon is directed primarily toward acid fermentation, whereas on polysaccharide substrates such as starch, carbon is directed toward exopolysaccharide synthesis.