Effect of co-culture of Bacillus tropicus and Lactococcus lactis on organic acid metabolism

Taking a strain of propionic acid-producing Bacillus tropicus R3 and a strain of lactic acid-producing Lactococcus lactis Z5 as the research objects, the differences in growth and metabolism of the two strains under pure culture and co-culture conditions were compared, and the effects of strain Z5 on the growth and metabolism of strain R3 were explored using glucose and lactic acid as carbon sources. The results showed that under the pure culture conditions, the strain R3 had the highest propionic acid yield (1.793 g/L) when the mass concentration of lactic acid was 10 g/L, while strain Z5 had the highest lactic acid yield (9.75 g/L) when the mass concentration of glucose was 30 g/L. Under co-culture conditions, the lactic acid was only generated by glucose metabolism with strain Z5, while propionic acid was only generated by glucose metabolism with strain R3. The results of the effects of strain Z5 on the growth and metabolism of strain R3 indicated that when strain Z5 and R3 were inoculated simultaneously, the growth advantage of strain Z5 exceeded that of strain R3, lactic acid generated by glucose metabolism with strain Z5 could provide substrate for strain R3 to metabolize and generate propionic acid and succinic acid.