초록
<P> Lactic acid, an attractive, renewable chemical for production of biobased plastics (polylactic acid, PLA), is currently commercially produced from food-based sources of sugar. Pure optical isomers of lactate needed for PLA are typically produced by microbial fermentation of sugars at temperatures below 40 °C. <I>Bacillus coagulans</I> produces L(+)-lactate as a primary fermentation product and grows optimally at 50 °C and pH 5, conditions that are optimal for activity of commercial fungal cellulases. This strain was engineered to produce D(−)-lactate by deleting the native <I>ldh</I> (L-lactate dehydrogenase) and <I>alsS</I> (acetolactate synthase) genes to impede anaerobic growth, followed by growth-based selection to isolate suppressor mutants that restored growth. One of these, strain QZ19, produced about 90 g L <SUP>-1</SUP> of optically pure D(−)-lactic acid from glucose in &lt; 48 h. The new source of D-lactate dehydrogenase (D-LDH) activity was identified as a mutated form of glycerol dehydrogenase (GlyDH; D121N and F245S) that was produced at high levels as a result of a third mutation (insertion sequence). Although the native GlyDH had no detectable activity with pyruvate, the mutated GlyDH had a D-LDH specific activity of 0.8 <I>μ</I> moles min <SUP>-1</SUP> (mg protein) <SUP>-1</SUP> . By using QZ19 for simultaneous saccharification and fermentation of cellulose to D-lactate (50 °C and pH 5.0), the cellulase usage could be reduced to 1/3 that required for equivalent fermentations by mesophilic lactic acid bacteria. Together, the native <I>B. coagulans</I> and the QZ19 derivative can be used to produce either L(+) or D(−) optical isomers of lactic acid (respectively) at high titers and yields from nonfood carbohydrates. </P>