Probiotic Yeasts and Bacteria: A Complementary Approach to Gut Health

Probiotics are live microorganisms that, when administered in adequate amounts, confer a health benefit to the host. Research has traditionally focused on bacterial species, particularly Lacticaseibacillus and Bifidobacterium, whose mechanisms are well-characterised. However, probiotic yeasts represent a functionally distinct category, with Saccharomyces boulardii as the most studied representative and Kluyveromyces marxianus as an emerging candidate. Bacterial and yeast probiotics differ in cell biology, antibiotic susceptibility, immune receptor engagement, and metabolic output. Bacterial strains provide mucosal adhesion and direct short-chain fatty acid (SCFA) generation but are susceptible to antibacterial therapy. Yeast strains are intrinsically antibiotic-resistant, offer enzymatic activities such as β-galactosidase and glycosidases, and exert indirect bifidogenic effects that amplify luminal SCFA concentrations beyond what either category achieves alone. These non-overlapping profiles support a complementary rather than competitive conceptualisation of their use. When combined with prebiotic substrates such as fructooligosaccharides or galactooligosaccharides, this integrated approach addresses the principal axes of intestinal homeostasis more comprehensively than any single-organism intervention. Prospective randomised trials are needed to determine whether this mechanistic complementarity translates into additive or synergistic clinical benefit. This review synthesises the available evidence and proposes a framework for their complementary use. The proposed framework is hypothesis-generating: it rests largely on mechanistic and preclinical data, and its clinical validity requires confirmation in adequately powered controlled trials.