Orally administered probiotics encounter various challenges on their journey through the mouth, stomach, intestine and colon. The health benefits of probiotics are diminished mainly due to the substantial reduction of viable probiotic bacteria under the harsh conditions in the gastrointestinal tract and the colonization resistance caused by commensal bacteria. In this review, we illustrate the factors affecting probiotic viability and their mucoadhesive properties through their journey in the gastrointestinal tract, including a discussion on various mucosadhesion-related proteins on the probiotic cell surface which facilitate colonization.

The low viability during gastrointestinal transit and poor mucoadhesion considerably limits the effectiveness of Ligilactobacillus salivarius Li01 (Li01) in regulating gut microbiota and alleviating inflammatory bowel disease (IBD). In this study, a delivery system was designed through layer-by-layer (LbL) encapsulating a single Li01cell with chitosan and alginate. The layers were strengthened by cross-linking to form a firm and mucoadhesive shell (~10 nm thickness) covering the bacterial cell. The LbL Li01 displayed improved viability under simulated gastrointestinal conditions and mucoadhesive function. Almost no cells could be detected among the free Li01 after 2 h incubation in digestive fluids, while for LbL Li01, the total reduction was around 3 log CFU/mL and the viable number of cells remained above 6 log CFU/mL. Besides, a 5-fold increase in the value of rupture length and a two-fold increase in the number of peaks were found in the (bacteria-mucin) adhesion curves of LbL Li01, compared to those of free Li01. Oral administration with LbL Li01 on colitis mice facilitated intestinal barrier recovery and restoration of the gut microbiota. The improved functionality of Li01 by LbL encapsulation could increase the potential for the probiotic to be used in clinical applications to treat IBD; this should be explored in future studies.

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Gastrointestinal disease is characterized by gastrointestinal dysfunction with dysbiosis of the microbiome. Probiotics may act as biological agents in treating gastrointestinal diseases through modifying gut microbiota. However, several challenges, including safety, stress resistance, postcolonization quantification, and evaluation models, may hinder the application of probiotics in gastrointestinal diseases. This review introduces the emerging methods for delivering probiotics as well as available materials. Furthermore, we elucidated bacteriocins and their role in helping probiotics obtain a competitive advantage over other strains and challenges of large-scale application. Bacteriocins produced by probiotics also showed promising efficacy in gastrointestinal diseases including the capacity of immune stimulation, intestinal barrier protection, and cytotoxicity against intestinal tumorigenesis. For the quantification of probiotics in complex microbiomes and evaluation methods of probiotic encapsulated delivery systems, recent fluorescent labeling technology and various in vitro and in vivo models were also reviewed. Given the widespread use of probiotic agents in the microecological therapy of gastrointestinal diseases, further understanding of the multiple challenges of probiotic application and the updated methods to improve the colonization and evaluation system of probiotics is of great significance for probiotics as live biotherapeutics.