Probiotics are typically marketed as foods and dietary supplements, categories for products intended to maintain health in generally healthy populations and which, unlike drugs, cannot claim to treat or cure disease. This review addresses the existing evidence that probiotics are beneficial to healthy people. Our approach was to perform a descriptive review of efficacy evidence that probiotics can prevent urinary, vaginal, gastrointestinal, and respiratory infections, and improve risk factors associated with cardiovascular health or reduce antibiotic use. Other endpoints such as mental, dental, or immune health were not specifically addressed. We concluded that there is sufficient evidence of efficacy and safety for clinicians and consumers to consider using specific probiotics for some indications - such as the use of probiotics to support gut function during antibiotic use or to reduce the risk of respiratory tract infections - for certain people. However, we did not find a sufficiently high level of evidence to support unconditional, population-wide recommendations for other preventive endpoints we reviewed for healthy people. Although evidence for some indications is suggestive of the preventive benefits of probiotics, additional research is needed.

Comprehension of underlying mechanisms of probiotic action will support rationale selection of probiotic strains and targeted clinical study design with a higher likelihood of success. This will consequently contribute to better substantiation of health claims. Here, we aim to provide a perspective from a microbiology point of view that such comprehensive understanding is not straightforward. We show examples of well-documented probiotic effector molecules in Lactobacillus and Bifidobacterium strains, including surface-located molecules such as specific pili, S-layer proteins, exopolysaccharides, muropeptides, as well as more widely produced metabolites such as tryptophan-related and histamine-related metabolites, CpG-rich DNA, and various enzymes such as lactase and bile salt hydrolases. We also present recent advances in genetic tool development, microbiome analyses and model systems, as well as perspectives on how the field could further progress. This opinion is based on a discussion group organized at the annual meeting of the International Scientific Association on Probiotics and Prebiotics (ISAPP) in June 2017.