Research Advances Understanding and Safety of *Saccharomyces boulardii* for Therapeutic Applications

Groundbreaking Studies Uncover New Potential and Safety Enhancements for Probiotic Yeast Saccharomyces boulardii

Researchers at North Carolina State University and an international team have conducted separate but complementary studies into the Saccharomyces boulardii (Sb) yeast, revealing significant advancements in its therapeutic potential and safety profile. These investigations pave the way for a new generation of probiotic and drug-delivery solutions.

One investigation focused on understanding the yeast's behavior within the gut to enhance its capacity for therapeutic drug delivery. Concurrently, another research effort identified specific genetic modifications to reduce the yeast's potential for infection, aiming to improve its safety profile for vulnerable populations while maintaining its probiotic properties.

Enhancing Therapeutic Drug Delivery

A study led by North Carolina State University researchers provides critical insights into how Saccharomyces boulardii yeast cells behave in the gut. This research aims to facilitate the development of new yeast strains capable of producing therapeutic drugs more efficiently for specific diseases.

Nathan Crook, a corresponding author and associate professor of chemical and biomolecular engineering at NC State, highlighted the promise of yeast as a drug-delivery platform. Previous work had shown that modified yeast cells could produce molecules to reduce inflammation or combat disease in the gut. The current study delved into the underlying mechanisms, focusing on gene expression and nutrient consumption, to boost the efficiency and effectiveness of yeast as drug-delivery vehicles.

Researchers selected Saccharomyces boulardii (Sb) yeast due to its established safety profile and commercial use as a probiotic. An unmodified, commercially available strain of Sb yeast was introduced into laboratory mice that had been raised to be germ-free, meaning they lacked a gut microbiome.

Fecal and intestinal samples were collected from the mice. A combination of sampling and analytical techniques was then used to measure the RNA produced by the yeast cells during their transit through the mice's guts. The germ-free environment was crucial, as it simplified the identification of yeast-specific RNA without interference from other microbes.

The study yielded several important discoveries regarding Sb yeast's behavior in the gut:

• Gene Activation: The study identified specific genes in Sb yeast that exhibited increased activity in the gut environment compared to other settings. These "promoter" sections of DNA could be targeted as on-switches to control the production of therapeutic molecules by engineered yeast cells, potentially improving production efficiency in drug delivery applications.
• Safety Profile: Significantly, genes associated with potentially pathogenic behavior in the yeast were not activated while in the gut. This finding reinforces and supports the established safety profile of Sb yeast, particularly when used as a probiotic.
• Nutrient Utilization: Gene activation patterns indicated that the gut environment was not rich in nutrients for the yeast. The yeast cells appeared to primarily digest lipids rather than carbohydrates for energy. This suggests that modifying yeast cells to better utilize complex carbohydrates in the gut could significantly enhance their energy supply and production efficiency for therapeutic compounds.

The paper, titled 'Transcriptomic Responses of Saccharomyces boulardii to the Germ-Free Mouse Gut,' was published in the journal BMC Genomics. Co-lead authors included Genan Wang and Deniz Durmusoglu, both affiliated with NC State.

Improving Probiotic Safety Through Genetic Modification

An international team of researchers, including those from North Carolina State University, focused on modifying Saccharomyces boulardii to improve its safety. This effort aims to make the probiotic suitable for vulnerable populations, such as immunocompromised individuals, older adults, and infants. Testing in an animal model indicated that the modified yeast demonstrated a reduced likelihood of causing infection compared to unmodified strains.

Saccharomyces boulardii is widely available commercially as a probiotic, recognized for its beneficial effects on gut health. However, there have been rare but severe instances of bloodstream infections reported in vulnerable patient groups—including immunocompromised individuals, infants, and the elderly—which have been linked to probiotic usage. The research was driven by the critical need to identify factors contributing to these infections and to explore genetic modifications that could reduce the yeast's virulence.

Researchers utilized Saccharomyces boulardii isolates from diverse sources, including commercial probiotic products and human patient samples (some from bloodstream infections). These isolates were used to infect immunosuppressed mice to determine their respective virulence levels. Sublineages from these mice were subsequently tested for their responses to various stress factors to identify adaptations associated with increased virulence.

A crucial finding was that the isolates exhibiting the highest virulence in the mouse model also demonstrated the highest tolerance to osmotic stress, indicating a superior survival capability in high-salt environments. The study then focused on two genes, ENA1 and NHA1, which are known to contribute to osmotic stress tolerance in yeast.

The researchers genetically edited both commercial and clinical Saccharomyces boulardii isolates to delete the ENA1 and NHA1 genes. While deleting NHA1 showed minimal impact on virulence, deleting ENA1 resulted in a significant reduction in virulence.

In compelling experiments, mice infected with a virulent yeast isolate saw their survival rate increase dramatically from 30-40% to 100% over a six-day period after the ENA1 gene was deleted. Additionally, deleting ENA1 impaired the growth of yeast strains when exposed to osmotic stress, thereby establishing a clear correlation between osmotic stress tolerance and virulence.

Further tests in a mouse model and in vitro antimicrobial assays indicated that the genetically modified Saccharomyces boulardii maintained its probiotic effectiveness. It was found to inhibit the growth of pathogenic bacteria species common in immunosuppressed patients and demonstrated similar survival rates in the gut compared to commercial probiotic lines. This suggests that the probiotic qualities of Saccharomyces boulardii are not significantly affected by the deletion of genes associated with osmotic stress, opening the door for safer probiotic applications.

Collaborative Efforts and Future Outlook

Both research efforts underscore the dynamic and ongoing development of Saccharomyces boulardii for therapeutic applications. The authors of the NC State study on yeast gene behavior have already filed patent applications and invention disclosures related to the engineering of probiotic yeast. Separately, an international PCT patent application for the commercial application of findings from the virulence study has been submitted by the University of Debrecen and North Carolina State University.

The safety-focused study indicates significant potential for developing engineered probiotic therapies that could be safe for immunocompromised patients, for whom current probiotic treatments may not be a viable or safe option. These complementary findings offer a promising future for enhanced and safer therapeutic applications of Saccharomyces boulardii.