Enantioselective lactic acid production by an Enterococcus faecium strain showing potential in agro-industrial waste bioconversion: physiological and proteomic studies

The growing demand of biodegradable plastic polymers is increasing the industrial need of enantiospe-cific l-lactic acid (l-LA), the building block to produce polylactides. The most suitable industrial strategyto obtain high amounts of LA is the microbial fermentation of fruit and vegetable wastes by lactic acidbacteria (LAB). In this paper seven LAB strains from our laboratory collection, were screened for their abil-ity to produce the highest amount of pure l-LA. A strain of Enterococcus faecium (LLAA-1) was selectedand retained for further investigations. E. faecium LLAA-1 was grown in different culture media supple-mented with the most abundant sugars present in agricultural wastes (i.e., glucose, fructose, cellobioseand xylose) and its ability to metabolize them to l-LA was evaluated. All tested sugars proved to begood carbon sources for the selected strain, except for xylose, which resulted in unsatisfactory biomassand LA production. Growth under aerobic conditions further stimulated l-LA production in fructosesupplemented cultures with respect to anoxic-grown cultures. Proteomic profiles of E. faecium LLAA-1 grown in aerobiosis and anoxia were compared by means of two-dimensional electrophoresis followedby MALDI-TOF mass spectrometry. Seventeen proteins belonging to three main functional groups weredifferentially expressed: the biosynthesis of 6 proteins was up-regulated in aerobic-grown cultures while11 proteins were biosynthesized in higher amounts in anoxia. The de novo biosynthesis of the f-subunitof alkyl hydroperoxide reductase involved in the re-oxidation of NADH seems the key element of theglobal re-arrangement of E. faecium LLAA-1 metabolism under aerobic conditions. An improved oxidativecatabolism of proteinaceous substrates (i.e., protein hydrolisates) seems the main phenomenon allowingboth higher biomass growth and improved LA production under these conditions.