This essay is written by Jongsik Jon Chun, a professor at the Department of Biological Sciences, Seoul National University and translated by ChunLab staffs. The author is also actively engaged in R&D as a major board member at ‘ChunLab, Inc.’
Beginning with the story of microbes within us
As I come across a diverse audience in my lectures, I realize that the ‘microbiome’ is still an unfamiliar concept to many, including the general public. Interestingly, an hour of discussion about what constitutes human microbiome and potential impact it may have on human health and disease is more than enough to turn them into microbiome enthusiasts.
The microbiome is mostly beneficial for us: they help digest our food, regulate our immune system, protect us from pathogen colonization, and produce vitamins for us. However, an imbalance to our microbiome, known as dysbiosis, are closely associated with a variety of human diseases: obesity, asthma, diabetes, rheumatoid arthritis, dementia, autism, etc. The wealth of associations sometimes makes the microbiome seem like science fiction or yet another quasi-science. However, thousands of published papers prove that the interplay between microbiota and human beings is undoubtedly real.
Like most other developed nations, we as Koreans have experienced ‘Industrialization’ a period of explosive economic growth leading to a rapid change in lifestyle. While our genes have remained immobile during this relatively short time frame, the types of microbes inhabiting our body have gone through major shifts. Such changes in microbial composition (aka, dysbiosis) have led to the increased prevalence of the diseases mentioned above. Ironically, excessive food and practice of sanitation and hygiene also increased the disease risks.
The microbiome is a ‘symbiotic’ ecosystem. Symbiosis is a relationship of co-existence through mutual benefit. So, what do we give to our microbiota? And what do we take from it? I would like to discuss the answers to these questions in this series about the Microbiome. For now, let’s focus on the negative impacts on human health when this symbiosis turn in the wrong direction. I wonder if numerous human diseases such as obesity, diabetes, metabolic diseases, mental disorder, autism and dementia are the result from lack of such symbiotic relationships between us and our microbiomes.
Each individual has a very unique microbial ecosystem that is not only different from that of other individuals, but also from that of one’s past itself. I find it very interesting that we have these microbial friends inside of our body. We could prevent number of diseases if we keep a healthy symbiotic relationship with our microbial friends. Likewise, I believe Bill Gates, the founder of Microsoft, has become a microbiome enthusiast for these reasons.
As today is the first day of my Microbiome series, I’d like to introduce several facts regarding the microbiome.
‘Microbiome Fact Check’
– Members of the microbiota
Sometimes used interchangeably, the microbiome refers to the collection of genomes from all the microorganisms in an environment. Microbiota, on the other hand, usually refers to specific microorganisms, including bacteria, archaea, fungi, protists, and viruses. All five of these types of microbes live on and within the human body. Although most of the microbes are beneficial to us, most of the research into the microbiome is about germs or bacteria, especially the one’s which causes disease.
– Microbiota population in human body
The number of microbial cells in our body outnumbers our own cells. Traditionally, the ratio between the number of microbial cells and human cells was thought to be 10:1, but recent studies have predicted that ratio is more likely to be 1:112. According to a recent research, there are about 38 trillion microbial cells live on and within an average adult human body weighing about 70kg. There exist 30 trillion human cells in our body, indicating that we have about 20% less human cells than the number of microbial cells inhabiting our body3.
– Weight of the gut microbiota
Each person’s microbiota are estimated to weigh 200 grams. The reason behind its lightness despite a cell count amounting to 38 trillion, is because each microbial cell is relatively smaller and less heavy than the human cell.
– Origin of the microbiome
We can divide the microbes found in our body into two types; the ‘tourists/transient’ that visit and leave soon after, and the ‘residents’ who settle down within our body. For example, Lactobacillus is the source of Kimchi fermentation and thus classified as a ‘lactic acid producer.’ Lactic acid producers help improve our health but they are only part of our tourists or transient microbiota. In contrast, the unfamiliar Faecalibacterium that accounts for ~20% of our microbiome is one of the residents.
We say that everyone is born germ-free. So, where did all these microbes come from? The microbes that form our microbiome, we get them from other humans we interact and the environment we surround. They may even exist in the air we breathe, water we drink, and they may also end up in our body from family members, friends, colleagues, and pets we routinely interact with, or anything we touch. Although members of the microbiota live in the same neighborhood, they all have different origins. For example, some might even be hitchhikers from your recent trip to New Delhi, India.
Newborn babies get their microbes first time during birth, from the mother’s birth canal, and then from breast milk. They also acquire microbes from their fathers, siblings, and caregivers.
– The history of symbiosis between human and microbe
A wealth of recent research highlights the intricate cross-talk between humans and microbes, and the important role it plays in human health and wellness. Then when did this all start? Based on the study that compared the gut microbiome between African wild chimpanzees and gorillas, researchers estimate that the symbiosis of man and microbe started at least 15 million years ago4.
Our common ancestor had a microbiome, and a considerable part of it has not yet left us and still lives inside us up to this day. The repertoire of genes in our current microbiome is a living record of our evolutionary history.
– The role of the microbiome
What role does the microbiome play in our bodies? To be frank, they live their lives ‘to the fullest’ just like us. Being packed like sardines inside the daily commute may not be so bad compared to being one of the 30 trillion microbes packed inside our large intestine.
Although microbes can be found in our skin and oral cavities, the majority of them live in our large intestine. Since our large intestine is a closed ecosystem, our microbes rely solely on us. The food that we cannot directly digest and the mucus we provide becomes their main source of food.
However, what is of importance to us is not what they ‘eat’, but rather what they ‘make’. The microbiome produces thousands of metabolites, which enter into our circulatory system through the intestinal wall. They then proceed with ease to reach any of our organs such as the liver or brain through this highway. Many well-reputed scholars of our time are working to elucidate the specific mechanisms that take place between microbial metabolites and our cells at each such body site.
Another critical role of the microbiome is to train our immune system. If such training should go wrong, autoimmune diseases, which are characterized by erroneous immune responses, may develop. There are numerous examples of autoimmune diseases, including atopic dermatitis, asthma, allergy, hyperthyroidism, etc. Such conditions are also related to the westernized lifestyle and improved hygiene. It is essential to guide the microbiome not to teach our immune system the wrong way.
In Conclusion
The symbiosis between microbes and human beings is complicated enough to be only partially explained even with the plethora of yearly scientific publications from the global research community. It is also a challenge to translate up-to-date scientific knowledge in a form that is readily understandable by the general public. This blog is dedicated to delivering novel concepts regarding the science of the microbiome in a palatable manner.
References
- 1.Rosner JL. Ten times more microbial cells than body cells in humans? Microbe. https://www.asmscience.org/content/journal/microbe/10.1128/microbe.9.47.2. Published 2014.
- 2.Sender R, Fuchs S, Milo R. Are We Really Vastly Outnumbered? Revisiting the Ratio of Bacterial to Host Cells in Humans. Cell. January 2016:337-340. doi:10.1016/j.cell.2016.01.013
- 3.Sender R, Fuchs S, Milo R. Revised Estimates for the Number of Human and Bacteria Cells in the Body. PLoS Biol. August 2016:e1002533. doi:10.1371/journal.pbio.1002533
- 4.Moeller AH, Caro-Quintero A, Mjungu D, et al. Cospeciation of gut microbiota with hominids. Science. July 2016:380-382. doi:10.1126/science.aaf3951