Researchers have modified a probiotic yeast to make it safer for use by immunocompromised people, older adults and infants.
The study, published in Nature's open-access journal Communications Biology, tested the modified yeast in an animal model and found that it was less likely to cause infection than unmodified strains of the same organism.
"The yeast we're working with is called Saccharomyces boulardii, which is marketed as a probiotic to promote gut health," said Alexandra Imre, first author of a paper on the work and a postdoctoral researcher at North Carolina State University.
However, there have been cases in which immunocompromised patients, infants and elderly people—who are especially vulnerable to infections—have gotten bloodstream infections due to probiotic usage.
"While these cases are rarely reported, they can be serious—even fatal"
"We wanted to learn more about what is contributing to these infections," Imre explained. "We were also wondering whether it is possible to genetically modify this yeast to make it less virulent, thus safer for immunosuppressed patient groups.”
The researchers examined S. boulardii yeast cell lines isolated from various sources, including commercial probiotics and human patients with bloodstream infections.
They infected immunosuppressed mice with these yeast cell lines and isolated several sublineages from the infected mice.
The researchers then tested these sublineages to assess their responses to stressors, aiming to identify adaptations associated with increased virulence.
"We found that the isolates that were most virulent in our mouse model were also the ones that were most tolerant to osmotic stress," Imre said.
The yeast lines that were most likely to increase virulence were also best able to survive in environments that have high concentrations of salts.
"We focused on two genes, ENA1 and NHA1, that play a substantial role in making the yeast more tolerant of osmotic stress."
The researchers genetically edited both the commercial and clinical S. boulardii isolates to delete the two genes, then repeated the virulence testing in mice and the stress testing.
"We found that deleting NHA1 made very little difference, but deleting ENA1 made a big difference," said Imre.
Before deleting ENA1, mice infected by the most virulent yeast isolate had a survival rate of 30-40%. After deleting ENA1, the survival rate increased to 100% during our six-day-long mouse infection experiment.
"We also found that deleting ENA1 resulted in impaired growth of the yeast strains when exposed to osmotic stress."
"Overall, our results showed that there is a clear correlation between osmotic stress tolerance and virulence," Imre added. "However, more research is going to be necessary to reveal the exact metabolic mechanisms behind this phenomenon; virulence attributes of emerging pathogens are under-researched."
The researchers also conducted tests using a mouse model and in vitro antimicrobial assays to determine whether modifications to S. boulardii would affect its probiotic efficacy.
They discovered that the genetically modified S. boulardii was as effective as commercially available probiotic strains in inhibiting the growth of bacterial species that commonly act as pathogens in immunosuppressed patients.
Additionally, the modified strain was equally capable of surviving in the gut, demonstrating its potential as a probiotic supplement.
"These are not exhaustive studies of probiotic efficacy, but our findings suggest the probiotic qualities of S. boulardii are not significantly impacted by deleting the genes associated with osmotic stress," said Nathan Crook, co-author of the paper and an associate professor of chemical and biomolecular engineering at NC State.
Many people with gut disease also have compromised immune systems, which means probiotic treatment is often not an option for them.
"This study demonstrates the potential for creating engineered probiotic therapies for use by immunocompromised patients. Further work is certainly necessary, but we're excited about this," he concluded.
The University of Debrecen and North Carolina State University have submitted an international PCT patent application for commercial application of the findings.