The Microbiome
What's the Fuss?
Over the past two decades, interest and research in the microbiome have grown substantially, with some even considering it our “last organ”. In the past decade alone, more than US$1.7 billion have been spent on medical research in this field. Researchers are particularly excited about the microbiome’s potential implications for chronic diseases, climate change, mental health, and various medical conditions.
What is the Microbiome?
Composition
The gut microbiota includes a vast collection of microorganisms residing within the gastrointestinal tract and on the skin. Numerous definitions of the microbiome have been published over the last few decades. One of the most cited definitions, by Lederberg, describes the microbiome within an ecological context: a community of commensal, symbiotic, and pathogenic microorganisms within a body space or other environment. Marchesi and Ravel's definition focuses on the genomes and microbial (and viral) gene expression patterns, along with the prevailing biotic and abiotic conditions. All these definitions imply the significant roles these organisms play within their environment.
The microbiota comprises all living members that form the microbiome. The inclusion of certain actors is a topic of debate. Bacteria, archaea, fungi, algae, and small protists are generally considered members of the microbiome. However, the inclusion of phages, viruses, plasmids, and mobile genetic elements remains controversial. These elements are often excluded from the microbiota definition because they are not considered living organisms.
In fact, there are more bacterial cells in the human body than human cells—roughly 40 trillion bacterial cells versus only 30 trillion human cells. Together, they function as an extra organ in the human body—a so-called “forgotten organ”—since these microbes have a collective metabolic activity equal to a virtual organ. The collective genome of the gut microbes, the gut microbiome, exceeds the human genome by over 100 times. Considering such enormous genetic potential, it is anticipated that the microbiota plays a role in virtually all physiological processes in the human body, including metabolic functions and immune homeostasis. The distinction between microbiota and microbiome is crucial: the microbiota refers specifically to the assemblage of living microorganisms, while the microbiome includes all members that interact with each other, live in the same habitat, and form their ecological niche together. This integrated community impacts their environment on a large scale, highlighting the significance of understanding both the microbiota and microbiome in ecological and health contexts.
Development
Each person has a unique network of microbiota, initially determined by their DNA. A person is first exposed to microorganisms as an infant, during delivery in the birth canal and through the mother’s breast milk. The specific microorganisms an infant encounters depend solely on the species found in the mother. Later on, environmental exposures and diet can alter one’s microbiome, influencing health outcomes either positively or negatively.
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