Composition and Interactions of the Oral–Gastrointestinal Microbiome Populations During Health, Disease, and Long-Duration Space Missions: A Narrative Review

Abstract

All forms of life on Earth are dependent on microbes. In vertebrates, the oral cavity and the gastrointestinal tract are colonized by large numbers of microorganisms, which includes species from many life forms: bacteria, fungi, archaea, and protozoa; this collection of microorganisms are commonly referred to as the microbiome. This article reviews the literature, providing a summary of oral and gastrointestinal microbial composition in health and during various disease states. Interactions are explored between microbes in the oral cavity and gastrointestinal tract. This study explores the influence of changed diet, lifestyle, and living conditions in order to examine the link between the oral and gastrointestinal microbiome and changes in their composition, and how this may affect human health. This study also investigates potential microbiome dysbiosis and disease causation in the space environment. The role of prebiotics, probiotics and postbiotics in oral health is discussed, with extension into the unexplored territory of probable oral and gastrointestinal bacterial population changes during long-duration (exportation class) space missions (ECSM).