High-throughput system for screening of high L-lactic acid-productivity strains in deep-well microtiter plates

For strain improvement, robust and scalable high-throughput cultivation systems as well as simple and rapid high-throughput detection methods are crucial. However, most of the screening methods for lactic acid bacteria (LAB) strains were conducted in shake flasks and detected by high-performance liquid chromatography (HPLC), making the screening program laborious, time-consuming and costly. In this study, an integrated strategy for high-throughput screening of high L-lactic acid-productivity strains by Bacillus coagulans in deep-well microtiter plates (MTPs) was developed. The good agreement of fermentation results obtained in the MTPs platform with shake flasks confirmed that 24-well U-bottom MTPs could well alternate shake flasks for cell cultivation as a scale-down tool. The high-throughput pH indicator (bromocresol green) and L-lactate oxidase (LOD) assays were subsequently developed to qualitatively and quantitatively analyze L-lactic acid concentration. Together with the color halos method, the pH indicator assay and LOD assay, the newly developed three-step screening strategy has greatly accelerated the screening process for LAB strains with low cost. As a result, two high L-lactic acid-productivity mutants, IH6 and IIIB5, were successfully screened out, which presented, respectively, 42.75 and 46.10 % higher productivities than that of the parent strain in a 5-L bioreactor.