초록
<P><B>Background</B></P><P>A thermal tolerant stereo-complex poly-lactic acid (SC-PLA) can be made by mixing Poly-D-lactic acid (PDLA) and poly-L-lactic acid (PLLA) at a defined ratio. This environmentally friendly biodegradable polymer could replace traditional recalcitrant petroleum-based plastics. To achieve this goal, however, it is imperative to produce optically pure lactic acid isomers using a cost-effective substrate such as cellulosic biomass. The roadblock of this process is that: 1) xylose derived from cellulosic biomass is un-fermentable by most lactic acid bacteria; 2) the glucose effect results in delayed and incomplete xylose fermentation. An alternative strain devoid of the glucose effect is needed to co-utilize both glucose and xylose for improved D-lactic acid production using a cellulosic biomass substrate.</P><P><B>Results</B></P><P>A previously engineered L-lactic acid <I>Escherichia coli</I> strain, WL204 (Δ<I>frdBC</I> Δ<I>ldhA</I> Δ<I>ackA</I> Δ<I>pflB</I> Δ<I>pdhR ::pflBp6-acEF-lpd</I> Δ<I>mgsA</I> Δ<I>adhE</I>, Δ<I>ldhA</I>::<I>ldhL</I>), was reengineered for production of D-lactic acid, by replacing the recombinant L-lactate dehydrogenase gene (<I>ldhL</I>) with a D-lactate dehydrogenase gene (<I>ldhA</I>). The glucose effect (catabolite repression) of the resulting strain, JH13, was eliminated by deletion of the <I>ptsG</I> gene which encodes for IIBC<SUP>glc</SUP> (a PTS enzyme for glucose transport). The derived strain, JH14, was metabolically evolved through serial transfers in screw-cap tubes containing glucose. The evolved strain, JH15, regained improved anaerobic cell growth using glucose. In fermentations using a mixture of glucose (50 g L<SUP>−1</SUP>) and xylose (50 g L<SUP>−1</SUP>), JH15 co-utilized both glucose and xylose, achieving an average sugar consumption rate of 1.04 g L<SUP>−1</SUP>h<SUP>−1</SUP>, a D-lactic acid titer of 83 g L<SUP>−1</SUP>, and a productivity of 0.86 g L<SUP>−1</SUP> h<SUP>−1</SUP>. This result represents a 46 % improved sugar consumption rate, a 26 % increased D-lactic acid titer, and a 48 % enhanced productivity, compared to that achieved by JH13.</P><P><B>Conclusions</B></P><P>These results demonstrated that JH15 has the potential for fermentative production of D-lactic acid using cellulosic biomass derived substrates, which contain a mixture of C6 and C5 sugars.</P>