Thermophilic Enterococcus faecium QU 50 enabled open repeated batch fermentation for l-lactic acid production from mixed sugars without carbon catabolite repression

Enterococcus faecium QU 50, a novel thermophilic l-lactic acid (LA) producing strain, was used in this study to ferment sugar mixtures into LA. Under the optimal fermentation conditions (50 °C, pH 6.5), strain QU 50 could ferment both mixed glucose/xylose sugars with relaxed CCR and mixed cellobiose/xylose sugars simultaneously without CCR to produce homo l-LA. The activity of enzymes related to xylose metabolism was also investigated. In the cells grown in a medium containing cellobiose/xylose, the activity of xylose isomerase and xylulose kinase, was 3.22 and 1.91 times higher, respectively, as compared to that of cells grown in a glucose/xylose medium. Strain QU 50 produced 70.8 g L^-1 of l-LA with a yield of 1.04 g g^-1 and a productivity of 2.95 g L^-1 h^-1 from simulated energy cane hydrolysate in batch fermentation. Immobilisation of strain QU 50 improved the operational stability of open repeated fermentation (three cycles), resulting in 61.1-64.3 g L^-1 of l-LA with a yield of 1.01-1.02 g g^-1 and a productivity of 3.22-3.82 g L^-1 h^-1. Thus, an efficient and cost-effective fermentation system was successfully established for l-LA production from sugar mixtures.