Abstract: In this study,wild-type L actobacillus rhamnosus was treated respectively by ultraviolet (UV) irradiation and nitrosoguanidine (NTG) to obtain mutant strains,and then they were subjected to recursive protoplast fusing. The acid tolerance and thermotolerance of the wild-type L actobacillus rhamnosus are improved by genome shuffling. A library that is more likely to yield positive results was created by using fusing the lethal protoplasts obtained from the strains treated by both UV irradiation and heat. After three rounds of genome shuffling,the strains that could grow at pH 3.6 were obtained. The L-lactic acid production is 3.4 times of that of the wild-type strain and the temperature is increased to 40 ℃. The results of shake-flask fermentation showed that the shuffled strains have the same pathway producing L-lactic acid as the wild-type one.

Key words: L-lactic acid, Lactobacillus rhamnosu, sgenome shuffling