There are trillions of tiny passengers living on and inside the human body in the form of bacteria, fungi and the third main domain of single-celled microbes known as the Archaea. Because of their extremely small size, some estimates suggest that there are between three and ten times as many microbial cells in the body than human cells.
There is mounting scientific evidence that these gut bacteria, or microbiota, perform vital tasks. Some scientists have even suggested that rather than the immune system controlling the microbiota, it is the microbiota that is to a large extent in control of the immune system. A healthy, diverse gut microbiota certainly seems to be a critical element in a healthy immune system, and a lack of diversity may play a role in autoimmune diseases such as diabetes, rheumatoid arthritis and multiple sclerosis, and perhaps even some cancers.
Some species of gut bacteria are known to promote the production of regulatory T-cells and can influence the process of inflammation. Gut microbiota certainly seem to be involved in inflammatory diseases of the gut, and there is a link – which may or may not be partly causal – with asthma and chronic obstructive pulmonary disease.
Animal studies have shown what happens to the developing immune system in a sterile environment. The immune system of germ-free mice does not mature normally and tissue homeostasis is never fully established. Lymphoid deficiencies occur in both the body’s mucous membranes and its systemic tissues, such as the lymph nodes and spleen. Germ-free mice, in other words, never develop a fully functioning immune system that can mount serious challenges to invading pathogens, yet they suffer from inflammation-induced tissue pathologies, such as asthma or colitis.
Scientists are now engaged on projects to decode the genome of the microbiota – the microbiome, the term coined by Joshua Lederberg in 2001. This microbiome research is already shedding light on the role it plays in keeping us healthy. It now seems we owe much to the fellow travellers living in our gut and that their health and wellbeing seems inextricably linked with our own.