By Paul Lawrence, Ph.D., and Joseph Ceccoli, Biocogent, LLC,
The term microbiome was first conceived approximately 22 years ago by Nobel Laureate Joshua Lederberg, Ph.D., who had a storied career as a research microbiologist. Since then, the use of this term has flourished across multiple disciplines and industries. But what is the precise definition of this cross-disciplinary term? It has also spawned an entire vocabulary unto its own with expressions such as microbiota, dysbiosis, biodiversity and biogeography.
Moreover, a significant investment in microbiome research endeavors has been predicated on the commonly described ratio of microbial to human cells; an estimate from 1972 put this average ratio at approximately 10:1.1 But how accurate is that figure? Does it hold up over time given the vast amounts of new data amassed in the last couple of decades?
Massive public endeavors such as the Human Microbiome Project (HMP) and the Earth Microbiome Project (EMP) have certainly contributed to a greater public awareness of the importance and implications of this field of study.2-4 The HMP was launched in two phases starting in 2007 with the stated goal of identifying and characterizing the microbes that reside on and within the human body that have impacts on human health.
The information gleaned from this effort was particularly targeted toward understanding if there was a correlative link between the constituents of the human microbiome and health issues associated with pregnancy, irritable bowel syndrome and the development of diabetes. The EMP has taken charge of an even more daunting task: identifying and cataloguing the planetary microbial community.
All too often, microbiome-related terms become conflated inappropriately and mischaracterized. Worse yet, some groups over-promise the degree of benefits resulting from application of microbiome-targeted products. Without proper correction, this could become a compounding problem for the cosmetic care industry.
Advances in scientific understanding of the human microbiome are published nearly every day, cumulatively adding to the knowledge base and demonstrating that science’s picture of the skin microbiome is far from complete. If consumer confidence is lost in the early stages of these endeavors, how can their faith and interest be restored when the remarkable complexity of the skin microbiome has been finally delineated?
It is the intention of this manuscript to dispel some of the myths surrounding the microbiome, properly define the nomenclature and instill an appreciation for the magnitude of the ongoing research and what has yet to be revealed.
Skin Microbiome vs. Skin Microbiota
Although often used interchangeably, the skin microbiome is technically distinct from the skin microbiota. Collectively, the microbes that take up residence on the skin represent the skin microbiota, which includes traditional bacteria, archaea, fungi and viruses. All of the genetic information contained in the genomes of these resident microbes is technically the skin microbiome. However, many institutions and individuals use the two terms somewhat interchangeably.
The distinction becomes important when returning to the question of the accuracy of the 10:1 ratio of microbial to human cells in the average human body. Revised estimates have changed the ratio to approximately 1.3:1 – nearly an order of magnitude difference. But if one considers the genetic contribution, the ratio returns to the previous 10:1 since roughly 80% of all human cells are red blood cells that, during maturation, dispense with their nuclei and by extension, their genetic complement.5, 6 From this perspective, the microbes that have taken up residence on and inside of our bodies provide a greater diversity of genetic information than our own human cells.
With the constant flurry of new findings in this arena of research, the cosmetic industry has leveraged multiple efforts to develop a series of products that attempt to modulate the microbiome to improve overall skin health. This is a complex proposition at best for a variety of reasons. First and foremost, our knowledge of the resident microbes on the skin remains incomplete, though substantial progress is being made toward the identification and characterization of the microbiota.
Of significance are the various so-called commensal microorganisms that are present on the skin and have either no impact or provide some type of benefit to the host. Many studies have determined that the three most abundant commensal microbes on the skin are two bacterial species: Cutibacterium acnes and Staphylococcus epidermidis; and multiple species of the fungal genera Malassezia.
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