From the late 19th century through the 20th century, scientists and physicians have characterized diseases that afflict humans as illnesses caused by germs. Contracting these diseases can be avoided by disinfecting. In the case that we do contract these pathogen caused diseases, doctors are trusted to identify the germ and propose a respective solution. Discovery and isolation of several microorganisms inside the human gut have occurred throughout the last century but it wasn’t until the first decade of the 21st century that the term “microbiome” was coined. The human microbiome is defined as the “full array of microorganisms that live on and in humans, and more specifically, the collection of microbial genomes that contribute to the broader genetic portrait or the metagenome of a human.”(1) Human microbiota are composed of microorganisms (bacteria, fungi, and viruses) that are harmful, seemingly irrelevant, or helpful to our health. The microorganisms thus live in mutualistic (beneficial to both parties), commensalistic (beneficial to one party, and not harmful to the other), or parasitic (beneficial to one party and harmful to another) symbiosis with humans. Thus, the “kill the germ, cure the disease” mentality, which has been ingrained in medicine simply does not apply to the microbiome. Diseases caused by malfunctions in the microbiome are almost never traced back to a single microbe, but instead are the result of some kind of systemic imbalance.
The Human Microbiome Project uses new genetic technologies to rapidly analyze genetic material of many microbes in human tissue, which, for the most part are unable to be cultured in laboratories(1). Differences in microbiota in humans have shown to have several disease-causing repercussions. There are correlations between an imbalanced microbiome and diseases such as obesity and rheumatoid arthritis(1). Perhaps the most interesting aspect of the microbiome is its connection with neuropsychiatric illnesses such as Obsessive Compulsive Disorder, Schizophrenia, Autism, etc. For example, in the study “The microbiota modulates gut physiology and behavioral abnormalities associated with autism,” Elaine Y. Hsiao et al. (2013) observed that even though autism is characterized by behavioral and mental disabilities, gastrointestinal symptoms are also often reported(1). In the study, corrected gut permeability in mice are shown to “ameliorate ASD [Autism Spectrum Deficiency] related defects in communicative, stereotypic, anxiety-like and sensorimotor behaviors”(1). Before extensive research of the microbiome had begun, scientists believed that the only way microorganisms and the brain interacted were when these pathogens would break the blood brain barrier, which often lead to fatal repercussions. However, now scientists have discovered that microorganisms in the gut release certain metabolites, which can change the activity of the cells in blood-brain barrier(1). Specifically, they often even interact with neurotransmitters, changing the behaviors of neurons.
Mice raised in completely sterile environments, lack the natural microbes in their gut that are normally found in wild mice; these lab mice have been shown to have lower levels of serotonin in their blood. These mice were less anxious which is not evolutionarily favorable for small prey(1). While the exact relationship between an imbalanced microbiome and lower anxiety has not been proven, the causality is in its beginning stages of research. So far, research regarding microbiota has been limited to mice models, and whether the relationships observed in studies cited above are also viable in humans is yet to be determined(1). Because of the intricacies of the microbiome, adding or removing microorganisms may have vast consequences on human health that scientists have yet to understand, but in the future, therapeutically shifting the human microbiome could be greatly useful in curing diseases(1).