It’s a Small, Small World: The Human Microbiome

Written by Erika Joloya and Edited by Rasheed Majzoub

In 1990, the Human Genome Project was started to map the entire genome, or the genetic sequence of human beings, focusing on the sections that are actively expressed and code for proteins. Researchers initially overestimated the number of protein-coding genes in the genome; approximately 20,000 were found versus the 100,000 predicted, revealing that the human genome is not that much larger than that of a fruit fly’s [1]. If the number of genes in the human genome is relatively modest, what is responsible for humans’ evolutionary and metabolic complexity? This question led scientists to research the microorganisms living within the human body; these microorganisms, known as microbial symbionts, live in and benefit off the nutrients and protection provided by the human body [1]. Scientists concluded that these microbial symbionts also contributed their own genomes to help humans develop specific traits they did not have to evolve themselves [1]. This new discovery inspired the launch of the Human Microbiome Project, whose goal was to understand the microbiome to a greater extent.  

The microbiome, often used interchangeably with the term “microbiota,” is simply defined as a collection of microbes living in a particular environment. Microbes are bacteria, viruses, fungi, and protozoa, and are so small that they are invisible to the naked human eye [2]. They make up only 2-3% of total human body weight despite vastly outnumbering human cell numbers [2]. While some microbes are harmless or beneficial, there are many common ones that can cause diseases such as malaria [3]. 

The skin is the largest and most complex organ in the human body. Composed of a system of cells, nerves, and glands, it protects against disease-causing pathogens and extreme environmental conditions [4]. The skin also has a mutualistic symbiotic relationship with a variety of microbes, or a relationship where both skin and microbe mutually benefit from each other. The microorganisms of the skin are fundamental to the skin’s physiology and immunity [5]. In fact, some of these microbes actively engage in protecting its host’s immunity by activating specific immune responses that protect against skin infections [5]. Therefore, having microbes on the skin is important to maintaining overall health and skin quality. In fact, studies suggest that a prolonged change in microbial communities may result in an altered state of skin that may lead to disease; for example, excessive hand washing may lead to damaging of the outer layer of the skin [6].  

The first line of defense in the skin is a layer of lipids that are vital to maintaining the hydration and flexibility of the skin. However, with frequent excessive hand washing with soap, the lipids on the superficial layers of the skin are washed off and not replenished in time to support the skin. Thus, lipids are not replenished in time and the skin is more prone to infection by pathogens entering the skin [6]. Furthermore, frequent washing may raise the skin’s pH from its originally low, acidic condition. This is particularly harmful to the skin because low pH contributes to its antibacterial characteristics [6]. Therefore, some companies are developing and advertising soaps “gentle” enough to use on the skin microbiome. Although the specifics of how the soap works are not clear, they claim that their microbiome soaps are gentle enough to not disrupt the microbiome balance or alter the skin’s pH. Research in regards to the scientific effectiveness of these “microbiome soaps” is lacking, and must therefore be looked into in order to establish scientific credibility. 

Scientific breakthroughs within the last few decades have inspired the foundations of new research. The Human Microbiome Project, in particular, prompted research on the human microbiome, and curiosity in the lives of microbial symbionts and the mechanisms by which they act on the body. Although more research must be done to fully understand the extent to which these microorganisms affect humans, one thing is clear: residential microbes significantly affect human health, despite the human microbiome being a small, small world. 

References:

1. Turnbaugh, P.J., Ley, R.E., Hamady, M., Fraser-Liggett, C.M., Knight, R., Gordon, J.I. (2007). The human microbiome project. Nature, 449:804–810. 
2. Macgill, Marcus. “What are the gut microbiota and human microbiome?” Medical News Today, Healthline Media, 2018, www.medicalnewstoday.com/articles/307998.php. 
3. “What are microbes?” Informed Health, Institute for Quality and Efficiency in Health Care, 2010, www.ncbi.nlm.nih.gov/books/NBK279387/.
4. Rosenthal, M., Goldberg, D., Aiello, A., Larson, E., Foxman, B. (2011). Skin Microbiota: Microbial Community Structure and its Potential Association with Health and Disease. Infection, Genetics, and Evolution, 11:839–848. 
5. Chen, Y.E., Fischbach, M.A., Belkaid, Y. (2018). Skin microbiota–host interactions. Nature, 553:427–436.
6. Larson, E. (1999). Skin hygiene and infection prevention: more of the same or different approaches? Clinical Infectious Diseases, 29:1287–1294.