The microbiome is the collection of all microbes, such as bacteria, fungi, viruses, and their genes, that naturally live on our bodies and inside us. Although microbes are so small that they require a microscope to see them, they contribute in big ways to human health and wellness. They protect us against pathogens, help our immune system develop, and enable us to digest food to produce energy.
Because the microbiome is a key interface between the body and the environment, these microbes can affect health in many ways and can even affect how we respond to certain environmental substances. Some microbes alter environmental substances in ways that make them more toxic, while others act as a buffer and make environmental substances less harmful.
How can the microbiome affect health?
The critical role of the microbiome is not surprising when considering that there are as many microbes as there are human cells, and each body site – for example, the gut, skin, and oral and nasal cavities – has a different community of microbes.
A person’s core microbiome is formed in the first years of life but can change over time in response to different factors including diet, medications, and environmental exposures.
Differences in the microbiome may lead to different health effects from environmental exposures and may also help determine individual susceptibility to certain illnesses. Environmental exposures can also disrupt a person’s microbiome in ways that could increase the likelihood of developing conditions such as diabetes, obesity, cardiovascular and neurological diseases, allergies, and inflammatory bowel disease. For example, specific changes in the gut microbiome have been linked to liver health. NIEHS-funded researchers and collaborators developed a rapid, low-cost tool that uses stool samples to detect microbial changes that can accurately diagnose liver fibrosis and cirrhosis.
Air pollution – NIEHS–funded research found breathing ultrafine particles (https://pubmed.ncbi.nlm.nih.gov/28211537/) , a component of air pollution, altered the gut microbiome and changed lipid metabolism in mice with atherosclerosis. Another study showed that exposure to traffic-related air pollution (https://pubmed.ncbi.nlm.nih.gov/34166366/) (TRAP) altered the respiratory microbiome in children.
Antimicrobials – A study found a profound effect from triclosan (https://pubmed.ncbi.nlm.nih.gov/28606169/) , a common ingredient in antimicrobial products, on the gut microbiome in mice. Mice that consumed triclosan through drinking water displayed an uptick in bacterial genes related to the stress response, antibiotic resistance, and heavy metal resistance.
Artificial sweeteners – Sucralose, an artificial sweetener, changes the gut microbiome (https://pubmed.ncbi.nlm.nih.gov/28790923/) in mice and may increase the risk of developing chronic inflammation. Another study found acesulfame potassium (https://pubmed.ncbi.nlm.nih.gov/28594855/), also an artificial sweetener, induced weight gain in male, but not female, mice.
Chronic stress – Chronic stress disturbs the gut microbiome (https://pubmed.ncbi.nlm.nih.gov/29531080/) in mice, triggering an immune response and promoting the development of colitis, a chronic digestive disease characterized by inflammation of the inner lining of the colon.
Diet – NIEHS researchers showed a high–fat diet affected the gut microbiome (https://genomebiology.biomedcentral.com/articles/10.1186/s13059-018-1389-1#Sec10) of mice in a way that predisposed them to gain weight and develop obesity.
Flame retardants – Early life exposure to types of flame retardants called polybrominated diphenyl ethers (https://pubmed.ncbi.nlm.nih.gov/34453844/) (PBDEs) and polychlorinated biphenyls (PCBs) can have a life-long impact on disease risk, which may be shaped by the gut microbiome.
Heavy metals – Arsenic exposure (https://pubmed.ncbi.nlm.nih.gov/28973555/) in mice changed the gut microbiome and altered molecular pathways in bacteria that are important to biological functions like DNA repair. A separate study suggested that the microbiome could protect (https://pubmed.ncbi.nlm.nih.gov/30575732/) mice from arsenic or methylmercury toxicity (https://pubmed.ncbi.nlm.nih.gov/33481013/) . In addition, research in a mouse model of Alzheimer’s disease demonstrated that exposure to cadmium (https://pubmed.ncbi.nlm.nih.gov/34912029/) altered an important communication pathway between the gut microbiome and the central nervous system called the gut-brain axis.
Infant Health – Birth mode, by C-section or natural birth, and what is eaten, formula or breast milk, during the first six weeks of life may affect the type of microbes in the gut microbiome of infants (https://pubmed.ncbi.nlm.nih.gov/26752321/) . The composition of the vaginal microbiome (https://pubmed.ncbi.nlm.nih.gov/34725359/) at birth can have lasting effects on offspring metabolism, immunity, and the brain.
Pesticides – Exposure to the widely used agricultural insecticide diazinon changed the gut microbiome (https://pubmed.ncbi.nlm.nih.gov/27203275/) of mice. These changes were more pronounced in male than female mice, providing insight into previously reported sex–specific effects of this toxicant on the nervous system.