Starch biotransformation by Lactococcus lactis: three strategies for process optimization

Microbial fermentation is increasingly used in the biotransformation of starch for sustainable production of chemicals and natural products. Lactococci have a rich array of metabolic pathways for the degradation of various carbohydrate sources but very few secrete α-amylase, which hydrolyzes starch into oligosaccharides. Here, the model strain Lactococcus lactis IL1403 was engineered to secrete α-amylase from Lactobacillus plantarum A6. This IL1403amy+ strain did not grow on starch and large quantities of unmetabolized maltooligosides accumulated in the growth medium, indicating imperfect starch hydrolysis. Three strategies were evaluated to overcome this bottleneck. First, a hybrid bacterial/enzyme process using added glucoamylase enabled 81% of the starch to be converted into lactic acid. Second, a synthetic consortium designed for labor division, with IL1403amy + and the non-conventional strain L. lactis EIP13A, capable of metabolizing a wide range of oligosaccharides, led to the complete conversion of starch into lactic acid, other organic acids and ethanol. Third, similar conversion efficiencies were obtained with a monoculture of EIP13A engineered to secrete α-amylase. This work demonstrates the potential of L. lactis for the biotransformation of starch into valuable products through different microbial approaches and opens the way to new strategies based on the diversity of natural strains to exploit other by-products.