title: 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.
authors.affiliation: Department of Biological Sciences, North Carolina State University, Raleigh, North Carolina, United States of America.
authors.forename: DeAnna E
authors.initials: DE
authors.lastname: Beasley
authors.affiliation: Department of Biology, Duke University, Durham, North Carolina, United States of America.
authors.forename: Amanda M
authors.initials: AM
authors.lastname: Koltz
authors.affiliation: Department of Anthropology, University of Colorado, Boulder, Colorado, United States of America.
authors.forename: Joanna E
authors.initials: JE
authors.lastname: Lambert
authors.affiliation: Department of Ecology and Evolutionary Biology, University of Colorado, Boulder, Colorado United States of America; Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, Colorado, United States of America.
authors.forename: Noah
authors.initials: N
authors.lastname: Fierer
authors.affiliation: Department of Biological Sciences, North Carolina State University, Raleigh, North Carolina, United States of America.
authors.forename: Rob R
authors.initials: RR
authors.lastname: Dunn
mesh_terms: D000818:Animals; D005075:Biological Evolution; D001717:Birds; D005516:Food Microbiology; D005744:Gastric Acid / Q000502:physiology*; D000069196:Gastrointestinal Microbiome; D060434:Herbivory; D054884:Host-Pathogen Interactions; D006801:Humans; D006863:Hydrogen-Ion Concentration; D008322:Mammals; D010802:Phylogeny; D013270:Stomach / Q000382:microbiology / Q000502:physiology; D013559:Symbiosis