초록
<P><B>Background</B></P><P>Optically pure <SMALL>D</SMALL>-lactic acid (≥ 99%) is an important precursor of polylactic acid. However, there are relatively few studies on <SMALL>D</SMALL>-lactic acid fermentation compared with the extensive investigation of <SMALL>L</SMALL>-lactic acid production. Most lactic acid producers are mesophilic organisms. Optically pure <SMALL>D</SMALL>-lactic acid produced at high temperature not only could reduce the costs of sterilization but also could inhibit the growth of other bacteria, such as <SMALL>L</SMALL>-lactic acid producers.</P><P><B>Results</B></P><P>Thermophilic <I>Bacillus coagulans</I> is an excellent producer of <SMALL>L</SMALL>-lactic acid with capable of growing at 50 °C. In our previous study, the roles of two <SMALL>L</SMALL>-lactic acid dehydrogenases have been demonstrated in <I>B. coagulans</I> DSM1. In this study, the function of another annotated possible <SMALL>L</SMALL>-lactate dehydrogenase gene (<I>ldhL3</I>) was verified to be leucine dehydrogenase with an activity of 0.16 units (μmol/min) per mg protein. Furthermore, the activity of native <SMALL>D</SMALL>-lactate dehydrogenase was too low to support efficient <SMALL>D</SMALL>-lactic acid production, even under the control of strong promoter. Finally, an engineered <I>B. coagulans</I> D-DSM1 strain with the capacity for efficient production of <SMALL>D</SMALL>-lactic acid was constructed by deletion of two <SMALL>L</SMALL>-lactate dehydrogenases genes (<I>ldhL1</I> and <I>ldhL2</I>) and insertion of the <SMALL>D</SMALL>-lactate dehydrogenase gene (<I>LdldhD</I>) from <I>Lactobacillus delbrueckii</I> subsp<I>. bulgaricus</I> DSM 20081 at the position of <I>ldhL1</I>.</P><P><B>Conclusions</B></P><P>This genetically engineered strain produced only <SMALL>D</SMALL>-lactic acid under non-sterilized condition, and finally 145 g/L of <SMALL>D</SMALL>-lactic acid was produced with an optical purity of 99.9% and a high yield of 0.98 g/g. This is the highest optically pure <SMALL>D</SMALL>-lactic acid titer produced by a thermophilic strain.</P><P><B>Electronic supplementary material</B></P><P>The online version of this article (10.1186/s12934-017-0827-1) contains supplementary material, which is available to authorized users.</P>