Genome-shuffling improved acid tolerance and L-lactic acid volumetric productivity in Lactobacillus rhamnosus

Genome shuffling is an efficient approach for the rapid improvement of industrially important microbial phenotypes. Here we improved the acid tolerance and volumetric productivity of an industrial strain Lactobacillus rhamnosus ATCC 11443 by genome shuffling. Five strains with subtle improvements in pH tolerance and volumetric productivity were obtained from the populations generated by ultraviolet irradiation and nitrosoguanidine mutagenesis, and then they were subjected for recursive protoplast fusion. A library that was more likely to yield positive colonies was created by fusing the lethal protoplasts obtained from both ultraviolet irradiation and heat treatments. After three rounds of genome shuffling, four strains that could grow at pH 3.6 were obtained. We observed 3.1- and 2.6-fold increases in lactic acid production and cell growth of the best performing at pH 3.8, respectively. The maximum volumetric productivity was 5.77+/-0.05 g/lh when fermented with 10% glucose under neutralizing condition with CaCO(3), which was 26.5+/-1.5% higher than the wild type.