The Evolution of Stomach Acidity and Its Relevance to the Human Microbiome

Abstract

Gastric acidity is likely a key factor shaping the diversity and composition of microbial communities found in the vertebrate gut. We conducted a systematic review to test the hypothesis that a key role of the vertebrate stomach is to maintain the gut microbial community by filtering out novel microbial taxa before they pass into the intestines. We propose that species feeding either on carrion or on organisms that are close phylogenetic relatives should require the most restrictive filter (measured as high stomach acidity) as protection from foreign microbes. Conversely, species feeding on a lower trophic level or on food that is distantly related to them (e.g. herbivores) should require the least restrictive filter, as the risk of pathogen exposure is lower. Comparisons of stomach acidity across trophic groups in mammal and bird taxa show that scavengers and carnivores have significantly higher stomach acidities compared to herbivores or carnivores feeding on phylogenetically distant prey such as insects or fish. In addition, we find when stomach acidity varies within species either naturally (with age) or in treatments such as bariatric surgery, the effects on gut bacterial pathogens and communities are in line with our hypothesis that the stomach acts as an ecological filter. Together these results highlight the importance of including measurements of gastric pH when investigating gut microbial dynamics within and across species.

Personalizing protein nourishment

“…Gastrin stimulates enterochromaffin-like (ECL) cells to release histamine, and histamine induces parietal cells to release gastric acid (Waldum et al, 2014). Gastric acid activates pepsinogen, denatures proteins and protects against pathogens (Martinsen et al, 2005; Beasley et al, 2015) (Figure 1A). Hypo-or achlorhydria (low or absent gastric acid, respectively) can result from a variety of conditions, including atrophic gastritis (Strickland and Mackay, 1973), gastrectomy (Hoya et al, 2009), acid suppression therapy (e.g., proton pump inhibitors, antacid medication) (Schubert, 2014) and Helicobacter pylori infection (Schubert, 2014).…”

Amphibian gut microbiota shifts differentially in community structure but converges on habitat-specific predicted functions

“…Host factors such as host genotype, stomach pH, mucins and antimicrobial peptides can impose selective filters thus shaping community composition131415161718. External factors such as diet and surrounding environment are also known to substantially influence microbial community composition1920212223242526272829303132.…”

The Gastric and Intestinal Microbiome: Role of Proton Pump Inhibitors

“…However, Beasley et al suggested that the evolution of stomach acidity plays an important role in maintaining balanced composition of gut microbiome in humans. [110] For example, it has been shown that one of the main factors determining gastric acidity is the diet habit of the host. Vultures which are considered obligate scavengers have one of the most hostile gastric environment (pH close to 0) which prevents colonization by many microbial strains.…”

“…There have been several proposed explanations why the stomach pH values in humans are similar to those of carrions: (1) carrion feeding in our direct ancestors and (2) natural selection favoring high gastric acidity aimed at prevention of infections caused by numerous fecal-oral pathogens. [110] Although increased gastric acidity prevents colonization by many pathogenic bacteria, the other side of the coin is that it also “filters out” many mutualistic microbes, thus preventing their colonization in certain patients (for example, after antibiotic use). [112]…”