The future of probiotics for disorders of the brain-gut axis

Probiotics, or at the very least products that might have probiotic properties, have been with us for decades, if not centuries, but it has only been in recent years that they have been subjected to serious scientific study. This surge in interest in probiotics has coincided with the era of the microbiome; as more and more is understood about the gut microbiota in health and disease, the therapeutic option of modulating the microbiota through the administration of probiotics has gained a more secure foundation. Regrettably, while a vast literature attests to the beneficial impact of probiotics in a variety of animal models and the mechanisms underlying such positive effects have been dissected in great detail, the data base on probiotics in man remains pretty slender. To make progress, a number of basic issues need to be addressed: strain characterization and other aspects of quality control need to be rigorously applied and additional steps such as dose optimization, definition of desired site of effect and tailoring of formulation accordingly accomplished before large scale trials, based on appropriately selected study endpoints and employing a clinically meaningful study duration, are embarked upon. Meantime, it is to be hoped that the regulatory climate will have been clarified and appropriate guidelines for the evaluation of probiotics, whether as food or drug, developed. Ultimately, the current terminology may have to be abandoned as evidence for biological and clinical activity for dead bacteria, bacterial components and bacterial products accumulates.