Human Health and the Microbiota: Interactions Between Gut Microbes, Hygiene, and The Immune System

Abstract

Background: In the past, much of the scientific research on microbes focused on mechanisms of infection and disease. This was not in vain, as we gained valuable knowledge about our immune system, as well as the ability to develop vaccines and antibiotics. However, the relationship between humans and microbes is complex. These species have been co-evolving since multicellular organisms evolved on Earth. Summary: Recently, it is beginning to be appreciated that the majority of relationships between humans and microbes are beneficial. From this follows an understanding that beneficial microbes are vital to the normal physiological development of our gut and immune system. This beneficial relationship between the human host and the multitude of microbial communities is well established. However, currently in the developed world epidemiological studies are showing dramatic increases in autoimmunity, allergies, and obesity. It is thus suggested that within westernized societies hygiene is altering the relationship between the gut and the human host in a way that makes humans susceptible to conditions not seen in less developed countries. This understanding advanced the “hygiene hypothesis,” and more recently the, “old friends hypothesis” and “disappearing microbiota hypothesis” as possible explanations for the observed epidemiological phenomena. What follows is a review of the relationship between gut microbes and the host’s immune system, with a focus on how hygiene (antibiotics, chlorination of water, etc.) is beginning to alter this relationship. This review concludes that a further understanding of how hygiene affects the relationship between humans and microbes will be crucial for developing effective therapies considerate of our microbial friends.