Saavedra JM. Use of Probiotics in Pediatrics: Rationale, Mechanisms of Action, and Practical Aspects. Nutrition in Clinical Practice, 2007;22(3):351-365.
The use of probiotics (ingested microbes that can modify intestinal microbial populations in a way that benefit the host) has moved from concept to actual demonstration of specific benefits by specific microorganisms for specific populations.
It is increasingly clear that these benefits to the host are mostly mediated by the profound effect that intestinal microflora (microbiota) have on gut barrier function and host immune response.
Intestinal bacteria are more numerous than the human cells of the host that harbors them. Despite having many potential pathogens in this microflora, humans do not routinely get infected. It is no coincidence that gut-associated immune tissue constitutes approximately 80% of all immunologically active cells in the human host.
The gut interacts with intestinal bacteria, both resident and ingested, to develop protective mechanisms (via improving gut barrier function and immune stimulation for defense) and appropriate, nonexaggerated responses (via immune modulation and immune tolerance) to support host health.
The mechanisms of this interaction between host and bacteria are increasingly being unraveled and in great part explain the clinical benefits that have been reported with specific probiotic bacteria by enhancing host defense mechanisms (such as for treatment and prevention of viral diarrhea and reducing risk of necrotizing enterocolitis), mitigating antibiotic-associated diarrhea, and modulating host immune response (such as in allergic disease).