Enhanced production of l‐lactic acid by Lactobacillus thermophilus SRZ50 mutant generated by high‐linear energy transfer heavy ion mutagenesis

Abstract

The aim of this study was to improve l‐lactic acid production of Lactobacillus thermophilus SRZ50. For this purpose, high efficient heavy‐ion mutagenesis technique was performed using SRZ50 as the original strain. To enhance the screening efficiency for high yield l‐lactic acid producers, a scale‐down from shake flask to microtiter plate was developed. The results showed that 24‐well U‐bottom MTPs could well alternate shake flasks for L. thermophilus cultivation as a scale‐down tool due to its a very good comparability to the shake flasks. Based on this microtiter plate screening method, two high l‐lactic acid productivity mutants, A59 and A69, were successfully screened out, which presented, respectively, 15.8 and 16.2% higher productivities than that of the original strain. Based on fed‐batch fermentation, the A69 mutant can accumulate 114.2 g/L l‐lactic acid at 96 h. Hence, the proposed traditional microbial breeding method with efficient high‐throughput screening assay was proved to be an appropriate strategy to obtain lactic acid‐overproducing strain.

Efficient l‐lactic acid production from purified sweet sorghum juice coupled with soybean hydrolysate as nitrogen source by Lactobacillus thermophilus A69 strain

“…Lactobacillus thermophilus A69 was screened via two‐round heavy ion irradiation and provided by the Biophysics Lab of the Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou, China . The strain was maintained on MRS agar slant at 4 °C.…”

“…Soybean hydrolysate was purchased from Solarbio Co., Ltd (Beijing, China). The seed medium composition was as described in a previous report …”

“…The fermentation process with slight modification was carried out as detailed in our previous report . The seed cultivation period by A69 was 8 h on a ZQLY‐180 N rotary incubator (Zhichu Co., Ltd, Shanghai, China) at 50 °C and 100 rpm.…”

“…The nitrogen source composition was 10 g L −1 peptone, 10 g L −1 beef extract and 5 g L −1 yeast extract. Other fermentation components were the same as in our previous report …”

“…Glucose was employed as carbon source. Other fermentation components were the same as in our previous report …”