Probiotics and gut microbiota: mechanistic insights into gut immune homeostasis through TLR pathway regulation

Consumption of probiotics as a useful functional food improves the host's wellbeing, and, when paired with prebiotics (indigestible dietary fibre/carbohydrate), often benefits the host through anaerobic fermentation. In this regard, many factors, including dietary ingredients and intestinal microbiota, influence the gut immune system's ability to sustain homeostasis. Thus, alteration of the intestinal microbiota is a potential alternative approach for maintaining health and preventing diseases. Probiotic formulations have been valued for "topping up your good bacteria" for their health benefits or for regulating the dysbiotic microbiota related to immunopathology. Intestinal epithelial cells (IECs) or immune cells associated with the lamina propria interact with probiotic bacteria through Toll-like receptors and induce an immune response by releasing different cytokines or chemokines. An additional mechanism of probiotic therapy is in enhancing the intestine's immunologic barrier, mainly through the intestinal response of immunoglobulin A and the mitigation of intestinal inflammatory responses, resulting in a gut-stabilising effect. The current commercially available probiotic products claim to balance the deregulated bacterial ecosystem, reduce disease risk prophylactically, or provide therapeutic benefits. This review focuses on the impact of probiotics on the underlying molecular events and signalling pathways involved in maintaining gut immune homeostasis and concludes by discussing perspectives. In this era of precision medicine and targeted therapies, more basic and clinical evidence is required to clarify the efficacy of probiotics through immune modulations that lead to preventing and treating disease.