Beyond MRS Broth: A soytone medium towards affordable culturing of South African vaginal Lactobacillaceae isolates

This study assesses a plant-based Soytone Medium as an alternative to the animal-derived standard de Man, Rogosa, and Sharpe (MRS) Broth for the cultivation of Lactobacillaceae. The application focuses on five isolates that have shown probiotic potential for bacterial vaginosis treatment. Cultivation was performed in 300 mL bench-scale bioreactors, monitored for cell density, pH, lactate production, and glucose consumption. The media's carbon and nitrogen concentrations and costing were quantified. Though the medium's carbon concentrations were identical, the Soytone Medium had a higher carbon-to-nitrogen ratio than MRS (8.1 vs. 6.6). Four strains achieved higher cell densities and maximum specific growth rates in the Soytone Medium. The greatest benefit was shown for L. crispatus 70.6PA, which had a 45% higher final cell density. A cost analysis showed that the Soytone Medium was 44% cheaper than MRS Broth. It was thus confirmed that the proposed plant-based Soytone Medium is a viable and less expensive alternative for Lactobacillaceae cultures in which exposure to animal products was also avoided. One-Sentence Summary: This study presents a plant-based Soytone Medium as a cost-effective alternative to standard MRS Broth for the high-density biomass cultivation of vaginal Lactobacillaceae, demonstrating enhanced growth performance and supporting their application in probiotic-based treatment of bacterial vaginosis in LMICs like South Africa.