Spore-Forming Probiotics and Their Mechanisms of Action: A Particular Focus on Alkalihalobacillus clausii

Gut microbiota play crucial roles in host health, including immune regulation, metabolism, and nutrient absorption. Its dysregulation has been linked to various diseases. When administered in sufficient amounts, probiotics can contribute to restoring the gut microbial balance and maintain homeostasis. Species of the genus Bacillus and related genera (Alkalihalobacillus and Heyndrickxia) have been extensively studied and commercialized as probiotics due to their ability to form endospores, the dormant cell forms that provide remarkable resistance to adverse conditions. Understanding the mechanisms of the action of spore-forming probiotics is essential for harnessing their therapeutic potential. This review explores the mechanisms underlying the action of spore-forming probiotics, with a special focus on Alkalihalobacillus clausii. Many beneficial properties such as resilience in extreme conditions, multiplication in the gut, gut adhesion, immunomodulatory effects, the production of bioactive and antimicrobial compounds, as well as efficacy in human health and diseases are extensively dissected. In parallel, this review underscores the limitations of spore-forming probiotics, focusing on safety concerns, issues related to dose standardization and quality control, as well as potential off-target effects and risks in immunocompromised individuals.