State‐of‐the‐Art Review on Probiotics, Synbiotics, and Microbial Metabolites: Molecular Mechanisms Shaping Host Immunity, Metabolic Health, and Chronic Disease Prevention

The human digestive system is first colonized at birth with many microorganisms that impact on the overall health of the individual via various metabolic, immune, and neuroendocrine pathways. Such microorganisms are referred to as probiotics, according to the FAO and WHO, and they exert effects on host and microbiota interaction primarily through strain-specific bioactive metabolites that act both locally and systemically. The current review will detail the various types of bioactive metabolites and their roles in regulating redox homeostasis via the reduction of reactive oxygen species (ROS) and increases in antioxidant defenses. These include catalase, glutathione peroxidase, and superoxide dismutase, which are believed to be responsible for reducing inflammation and improving epithelial barrier function. In addition to the traditional single-strain approach to probiotics, synbiotics represent an alternative strategy to improve microbial survival, functional stability, and microbiome homeostasis. Synbiotics consist of a combination of probiotic organisms and selective prebiotics that enhance the survivability and functional capacity of the individual strains and thus the overall functional resilience of the gut microbiome. Emerging evidence from mechanistic, experimental, and clinical studies demonstrates the increasing relevance of synbiotics in both therapeutic and translational contexts in the modern healthcare system.