A microbiome can be defined as all the microorganisms, e.g. bacteria and viruses, which occupy a specific niche or ecological location. For humans, our microbiome is the sum total of all the microorganisms that inhabit our bodies. The vast majority of work on the human microbiome has focused on the bacteria that co-inhabit our bodies. These studies have generated fascinating new information about our indigenous bacterial communities, as well as exciting and tantalizing new connections between our microbiome and our health.
One of the first things that stands out is the sheer number and diversity of the organisms in and on our bodies. Most estimates place the number of microbes at around 100 trillion, which is 10 times more than the number of cells in our bodies! Collectively these microbes add up to about 5 pounds of our body weight. More importantly, these microorganisms encode about 8 million genes compared to the paltry 20,000-25,000 genes in our own DNA. Together, the microbial genes plus our human genes are called the metagenome. What proteins these millions of genes produce and what affect these exogenous proteins have on our metabolism, immune system, and overall biology is being heavily researched; but, results and conclusions are still in the early stages of understanding.
One thing that stands out is the complexity of our bacterial communities. We begin to acquire bacteria immediately as we are born and pass through the vaginal canal replete with our mother’s unique microbial composition. From there we rapidly acquire organisms from food (including breast milk), water, the environment, and contact with other persons. A mature human microbiome contains 500-1000 species of bacteria with varying ratios and distributions of species at different anatomical sites, e.g. oral cavity versus the intestinal tract. Additionally, a person’s microbial signature is not static and can change over time in response to many factors such as illness, antibiotic usage, pregnancy, stress, and diet. There can also be striking variation between different individuals, and probing how personal differences in microbiome composition influence one’s health is a key research question.
It has long been known that our intestinal bacteria not only aid digestion, but also produce several critical nutrients such as the B vitamins known as B12, folate, and thiamine. What is more intriguing are the findings that the gut microbiome has been linked to a wide range of health conditions including diabetes, autism, anxiety, obesity, and immune function, just to name a few. While correlation does not equal causation, there is great enthusiasm in the scientific community that the microbiome plays a key role in overall health by impacting a variety of our biological processes. Importantly, understanding how the microbiome contributes to health and disease opens up new vistas of therapy. Manipulating the microbiome composition with drugs, probiotics, or bacterial transplants has already been attempted for some conditions, and this approach holds substantial promise for future therapies. Someday, having your microbiome analyzed may be a standard part of personalized medicine, just as individual genome sequencing is becoming more routine. I’ll follow up in later posts with examples of novel and exciting studies linking the microbiome to specific health issues.