Intestinal pH: a major driver of human gut microbiota composition and metabolism

In the human gastrointestinal tract, pH is a key factor in shaping gut microbial composition and activity, while also being influenced by microbial metabolism. pH varies substantially along the gastrointestinal tract within an individual and between different individuals due to a combination of host, diet, microbial and external factors. The importance of pH on microbiota composition and metabolic response has been widely explored over the past century. Here, we review the literature to explore the major physiological and dietary factors that influence pH along the gastrointestinal tract. From a microbial ecology perspective, we discuss how gastrointestinal pH affects microbiota composition and metabolism. We explore mechanisms by which pH can influence bacterial acid response systems, gene expression and the production of microbial metabolites important for health. Finally, we review the literature regarding the potential role of gastrointestinal pH in human diseases. We propose that we can advance our understanding of the gut microbiota in health and disease by considering gastrointestinal pH. We argue that pH-mediated gut microbial metabolic variation is highly important for predicting and manipulating metabolic output relevant to human health.