Monascus ruber as cell factory for lactic acid production at low pH

- A Monascus ruber strain was isolated able to grow at 150 g/l lactic acid at pH 2.8.
- NLDH genes were introduced and PDC genes were knocked out to evoke lactic acid production.
- Lactic acid consumption was stopped by knocking out CLDH genes.
- The sugar consumption rate was enhanced three-fold by evolutionary engineering.
- The final strain produced lactic acid to a titer of 190 g/l at pH 3.8 and 129 g/l at pH 2.8.

A Monascus ruber strain was isolated that was able to grow on mineral medium at high sugar concentrations and 175 g/l lactic acid at pH 2.8. Its genome and transcriptomes were sequenced and annotated. Genes encoding lactate dehydrogenase (LDH) were introduced to accomplish lactic acid production and two genes encoding pyruvate decarboxylase (PDC) were knocked out to subdue ethanol formation. The strain preferred lactic acid to glucose as carbon source, which hampered glucose consumption and therefore also lactic acid production. Lactic acid consumption was stopped by knocking out 4 cytochrome-dependent LDH (CLDH) genes, and evolutionary engineering was used to increase the glucose consumption rate. Application of this strain in a fed-batch fermentation resulted in a maximum lactic acid titer of 190 g/l at pH 3.8 and 129 g/l at pH 2.8, respectively 1.7 and 2.2 times higher than reported in literature before. Yield and productivity were on par with the best strains described in literature for lactic acid production at low pH.