The Human Microbiome

Outnumbered in Our Own Bodies

All humans are hosts for billions of bacteria, archea, and other microorganisms. There are a myriad of species living within us. In fact, there are between 500 and 1,000 species of bacteria living in most people’s intestinal tract alone, according to a Nature article. This is not even taking into account the many more species that live elsewhere on our bodies. In a way, we are outnumbered in our own bodies. According to Dr. Cramer, the ratio of human microbiota cells to actual human cells can be as large as 3 to 1! Despite the large number of non-human cells living on and in us, the human microbiota account for only about 1 kg of our weight. This is because of the very small size of the prokaryotes. Nonetheless, while this may seem like a small percentage of our total weight, there are many organs that are measured to have the same weight. But why do we have so many small friends hanging around inside us?

A New Organ

The human microbiome is vital to our health. Many bacteria in our gut produce compounds that we need and cannot produce on our own. For example, bacteria in our gut produce vitamin K, some vitamin Bs, and butyrate which we can then absorb. There are even studies that show our microbiota produce certain neurotransmitters which can stimulate the nervous system, communicate with our brain, and, as a result, contribute strongly to our mood. In addition, normal microbiota helps protect us from harmful pathogens by covering pathogen binding sites, competing for nutrients, and/or producing compounds that are toxic to pathogens. For example, Clostridioides dificile, a bacterium that causes potentially fatal colitis, can only infect humans if there is a decrease in competition from our normal microbiota, usually caused by antibiotic medications. For these reasons, many scientists consider the human microbiome to be its own organ.

Depression and the Microbiome

Many studies show that there is an association between a disturbed or unusual microbiome and depression. According to a Psychiatry Advisor article, “the gut-brain-microbiota axis is a bidirectional communication system allowing gut microbes to communicate with the brain and for the brain to then send signals to the gut”. This direct communication with the brain could mean that not only intestinal disturbances and diseases could affect long-term mood, but even the food we eat could affect our daily attitude. The same Psychiatry Advisor article reports that the majority of irritable bowel syndrome patients experience depression. Needless to say, a better understanding of our microbiome could lead to improved depression therapies. However, according to a Nature article, a major challenge is the incredible diversity in microbiota between “apparently healthy” individuals. In other words, what’s good for one person’s gut may not be the best for another’s. Nonetheless, research is ongoing, and someday we could be treating depression patients exclusively with probiotic smoothies!