Direct Conversion of Cellulose to L-lactic Acid by a Novel Thermophilic Caldicellulosiruptor Strain

Background: Consolidated bioprocessing (CBP) of lignocellulosic biomass to L-lactic acid using thermophilic cellulolytic/hemicellulolytic bacteria provides a promising solution for efficient lignocellulose conversion without the need for additional cellulolytic/hemicellulolytic enzymes. Most studies on the mesophilic and thermophilic CBP of lignocellulose to lactic acid concentrate on cultivation of non-cellulolytic mesophilic and thermophilic bacteria at temperatures of 30-55 °C with external addition of cellulases/hemicellulases for saccharification of substrates.

Results: L-Lactic acid was generated by fermenting microcrystalline cellulose or lignocellulosic substrates with a novel thermophilic anaerobic bacterium Caldicellulosiruptor sp. DIB 104C without adding externally produced cellulolytic/hemicellulolytic enzymes. Selection of this novel bacterium strain for lactic acid production is described as well as the adaptive evolution towards increasing the L-lactic acid concentration from 6 to 70 g/l on microcrystalline cellulose. The evolved strains grown on microcrystalline cellulose show a maximum lactic acid production rate of 1.0 g/l*h and a lactic acid ratio in the total organic fermentation products of 96 wt%. The enantiomeric purity of the L-lactic acid generated is 99.4%. In addition, the lactic acid production by these strains on several other types of cellulose and lignocellulosic feedstocks is also reported.

Conclusions: The evolved strains originating from Caldicellulosiruptor sp. DIB 104C were capable of producing unexpectedly large amounts of L-lactic acid from microcrystalline cellulose in fermenters. These strains produce L-lactic acid also from lignocellulosic feedstocks and thus represent an ideal starting point for development of a highly integrated commercial L-lactic acid production process from such feedstocks.