The human microbiome consists of microorganisms such as bacteria, archaea, and viruses inhabiting the skin, gastrointestinal, reproductive, and respiratory tracts.
These microbial populations are as diverse as comprising 500 -1000 bacterial species. With each of these species having thousands of genes, the collective bacterial genome in our body is more than an estimated 20,000 genes found within the human genome. With this large repertoire of genes and their products, the microbiome can affect processes going on in different cells of the host body. Several studies show how the microbiome has co-evolved with different components of the host immune system.
What is the role of gut microbiota in immunity?
Germ-free mice that have no microorganisms living in or on them have been reported to have several defects in their immune system. The resulting lack of immunity at different levels makes them more susceptible to infections. However, if microbiota from standard pathogen-free mice is transplanted into them, the defects in the immune system are corrected and maturation of the immune system takes place. These early studies indicated the role of the microbiome in the development of the robust immune system.
Our immune system can be divided into two major components – i) Innate Immunity- It is the immunity that we are born with and ii) Adaptive immunity – It is the immunity that is acquired or developed by an individual during their lifetime. The microbiome is involved in the development of both these components of immunity.
Different reports highlight the role of microbial species in promoting the development of specific immune cells. Bacteroides fragilis and other Bacteroides species such as Parabacteroides distasonis, Parabacteroides gordonii, Alistipes senegalensis, Parabacteroides johnsonii, Paraprevotella xylaniphila, and a few non -Bacteroides like Eubacterium limosum, Ruminococcaceae bacterium, Fusobacterium ulcerans, Phascolarctobacterium faecium, have been reported to induce certain cells of adaptive immunity in GF mice. Further gut commensals have been reported to activate dendritic cells of the immune system.
The metabolites produced by the microbiome appear to play an important role in manipulating the immune system. For example, the butyrate produced by gut microbes is essential for the survival and response of immune cells and can result in enhanced memory response. Butyrate and propionate produced by microbiota during fermentation of fibers can also induce immune cells through epigenetic alterations.
Human studies also show the importance of the gut microbiome in the maturation of the immune system. The microbiome of the gut varies drastically between newborns depending on their mode of delivery. The gut flora of neonates delivered via cesarean mostly consists of skin commensals like - (Staphylococcus, and Corynebacterium), while those born by normal vaginal delivery consist of microbes such as (Lactobacilli, Prevotella spp).
Interestingly, in the former case, there is an increased risk of autoimmune disorders like diabetes, asthma, allergies, and arthritis. Consumption of antibiotics in the early stages of life can also modify gut microbiome composition and has been reported to be associated with higher chances of developing inflammatory bowel disease (IBD), obesity, and cancer.
An interplay between the microbiome and the host immune system is a reflection of evolution for the benefit of both. Both are adapting together to changing conditions and it's not for the betterment of just one, but for both partnersDr. Anita Saini, Assistant Professor Microbiology, (Maharaja Agrasen University)
However, it should be kept in mind that most of the above-mentioned studies are associative and do not show causality between gut microbiome changes and diseases. While causal studies investigate whether two variables have a cause and effect relationship, association studies only evaluate if a change in one results in a change in the other (without establishing a cause and effect relationship).
Microbiome constantly undergoes fluctuations depending on food choices, age, consumption of probiotics, antibiotics, etc. Consequently, these environmental factors can impact body immunity through pathways involving the microbiome. Further microbiome and immune system can interact bidirectionally, with the microbiome and their metabolites helping in the development of the immune system and immune system in turn determining the composition of the microbiome.