This thesis examines the therapeutic potential of probiotic microorganisms, particularly Lactobacillus, Bifidobacterium, and Saccharomyces, emphasizing their capacity to survive gastrointestinal conditions and modulate the host microbiota. These strains demonstrate key functions including pathogen inhibition, immune regulation, and enhancement of epithelial barrier integrity. Clinical evidence supports their efficacy in managing gastrointestinal disorders, with additional roles emerging in gut-brain axis modulation through the production of neuroactive compounds. Recent oncology research highlights their capacity to improve immunotherapy outcomes and reduce treatment-related toxicities. Variability in clinical responses underscores the importance of strain specificity, host factors, and methodological consistency. Advancing probiotic-based interventions will require personalized strategies, standardized protocols, and integration of microbiome sequencing and metabolomic profiling.