Metabolic engineering strategies for consolidated production of lactic acid from lignocellulosic biomass

Lactic acid (LA) is one of the most desired molecules by the chemical industry. Current expansion of LA market is mainly driven by its application as building block for the synthesis of polylactide (PLA), that is, a family of biodegradable and biocompatible plastic polymers. PLA can potentially replace oil-derived polymers as general purpose plastic, but current LA prices fails to make PLA cost-competitive with traditional plastics. Nowadays, LA is mainly produced by fermentation of expensive starchy biomass. Hopefully, cheaper lignocellulosic feedstock could be used in future second-generation biorefinery processes. However, most efficient natural LA producers cannot ferment lignocellulose without prior biomass saccharification. Metabolic engineering may develop improved microorganisms that feature both efficient biomass hydrolysis and LA production, thus supporting consolidated bioprocessing (CBP), that is, one-pot fermentation, of lignocellulose to LA. CBP could dramatically reduce LA production cost, thus contributing to the expansion of more environmental sustainable plastics and commodity chemicals. This review presents an overview of “recombinant cellulolytic strategies”, mainly consisting in introducing cellulase systems in native producers of LA, and “native cellulolytic strategies” aimed at improving LA production in natural cellulolytic microorganisms. Issues and perspectives of these approaches will be discussed.