New selective media for isolation and enumeration of Lactobacillus rhamnosus and Streptococcus thermophilus

To improve the quality and function of fermented products, Lactobacillus rhamnosus and Streptococcus thermophilus are often used together. However, it was found that MRS agar medium was not efficient in distinguishing between L. rhamnosus LV108 and S. thermophilus grx02. Several modified MRS agar media were investigated in this study. MRS agar supplemented with sodium acetate could not inhibit the growth of S. thermophilus grx02. Though S. thermophilus grx02 did not grow on low pH MRS (pH 5.0), L. rhamnosus grew slowly on this medium, and its CFU count was lower than that on MRS. MRS without glucose (Gly-MRS) didn’t support the growth of S. thermophilus, but L. rhamnosus could produce well-defined colonies as it did on MRS, which were not influenced by the growth phases or culture media for the tested strains. In addition, when cocultured with L. rhamnosus LV108, the viable count of S. thermophilus grx02 could be enumerated by controlling the culture time on LM17. By restricting the carbon source and culture time, mixtures of L. rhamnosus LV108 and S. thermophilus grx02 could be enumerated rapidly and easily in 48 h (minimum 36 h), which could benefit the development, production and storage of probiotic dairy products containing these two species.