Extracellular proteins secreted by probiotic bacteria as mediators of effects that promote mucosa–bacteria interactions

During the last few years, a substantial body of scientific evidence has accumulated suggesting that certain surface-associated and extracellular components produced by probiotic bacteria could be responsible for some of their mechanisms of action. These bacterial components would be able to directly interact with the host mucosal cells; they include exopolysaccharides, bacteriocins, lipoteichoic acids and surface-associated and extracellular proteins. Extracellular proteins include proteins that are actively transported to the bacterial surroundings through the cytoplasmic membrane, as well as those that are simply shed from the bacterial surface. Compared to the other bacterial components, the interactive ability of extracellular proteins/peptides has been less extensively studied. In this review, current findings supporting an interaction between extracellular proteins/peptides produced by probiotic bacteria (strains of the genera Bifidobacterium, Lactobacillus and Escherichia) and host mucosal cells are discussed. Research needs and future trends are also considered.