You know, medicine is not an exact science, but we are learning all the time. Why, just fifty years ago, they thought a disease like your daughter's was caused by demonic possession or witchcraft. But nowadays we know that Isabelle is suffering from an imbalance of bodily humors, perhaps caused by a toad or a small dwarf living in her stomach. - Theodoric of York, Medieval Barber (Saturday Night Live)
While we do not share our bodies with small dwarfs, toads, or (literal) demons, we are not alone.

It has been said that “No man is an island.” While you may quibble that it should be “No one is an island,” we know what it means: We human beings depend on one another. We depend on each other, and we also depend on ecosystems to provide us with water and clean air—among other things.

There are other important ecosystems within us and on us.You are no island: no, you are more of a continent complete with colonists, invaders, battles for resources, and turf wars. And there are a lot more of “them” than there are of “you,” about one hundred trillion of them. As one article in the Economist put the idea, “…humans are not single organisms, but superorganisms made up of lots of smaller organisms working together.”
We have known for a long time that our guts harbor “good” bacteria (yogurt companies advertise about probiotics) and health officials caution against unnecessarily taking antibiotics which could harm good bacteria. These bacteria, it turns out, have evolved along with us (Homo sapiens) and are part of our being. And, in turn, our bacteria evolve within us, having numerous generations during a person’s lifespan, and adapting to changing conditions.

What is now coming out of research is how essential those bacteria are to our physical and mental health. For instance, on our skin, “Staphylococcus epidermidis fends off skin infection and enhances immunity,” the Economist article says. Maybe that antibacterial soap isn’t your best choice for healthy skin.

Researchers call the symbiotic relationship that microbes have with particular animals or plants a microbiome. The sheer magnitude and diversity of your microbiome is staggering. “The typical human is home to a vast array of microbes,” evolutionary biologist Olivia Judson wrote in the New York Times. “If you were to count them, you’d find that microbial cells outnumber your own by a factor of 10. On a cell-by-cell basis, then, you are only 10 percent human. For the rest, you are microbial.” A human being has 23,000 different genes. Our microbiome has almost 150 times that number, about three million genes.

In their proper places, microbiomes are truly symbiotic, a collaboration of human and micro-critter. We provide hospitable living conditions, and the microbes help break down foods for digestion, synthesize vitamins, and help our immune system. “The past few years have shown that having good relations with the 100 trillion bacteria which inhabit the gut is essential to human health,” reports an Economist article.

Inoculation with microbiota begins when we travel through the birth canal. Among other things, our new gut bacteria will “affect the wiring of nerves in the stress system, influencing how the body reacts to stress for the rest of its life,” writes Tom Siegfried in Science News. Our mothers’ influence, then, goes even further than we knew.

When they are not in their proper place or when unwanted bacteria come in, the results can be distressing, painful, or even deadly for us, the hosts. “If relations break down, hostile bacteria may invade and previously friendly ones may turn hostile," the Economist article said.  Researchers have linked off-kilter microbiomes to obesity, diabetes, heart disease, autism, and some autoimmune diseases. Clostridium difficile is a condition caused by the loss of beneficial bacteria.

Rejiggering some microbiomes appears to cure some diseases. "When things do go wrong, though," the Economist reported, "doses of corrective bacteria can make a difference.”

The method of delivery for healthy bacteria to the intestine is rather yucky. Yes, eating yogurt with probiotics can help people with irritable bowel syndrome, but pretty much everything else requires a fecal transplant—a “trans-poo-sion,” if you will. Gastroenterologist Thomas Borody says, “By implanting another person’s stool, that other person may contain bacteria which manufacture antibiotics. And this is the key: bacteria make chemicals that kill other bacteria. In fact, most antibiotics come from bacteria.” Fecal transplants can change the gut’s microbiome, and this changes our health.

Scientists have just begun to understand our microbiome’s interaction with us. For one thing, there is much to learn simply due to the number of these critters. “The adult human intestine contains trillions of bacteria, representing hundreds of species and thousands of subspecies,” one scientific abstract says. We are also at the beginning of this scientific process; a time that is analogous to when people knew willow tree bark relieved headaches but had not yet identified acetylsalicylic acid (the active ingredient in aspirin) as the reason.Our microbiomes and earth’s biomes (plants and animals found in particular habitats) have evolved and continue to evolve as conditions change. Understanding their complexities will help improve our lives. And, as always, more research is needed.

References/Further Reading

Dubner, S. (2011, March 4). Freakonomics. Retrieved June 6, 2013, from The Power of Poop:

The Economist. (2013, February 21). Evolution: History Repeating. Retrieved June 7, 2013, from The Economist:

The Economist. (2013, April 11). Microbes and men: Consumer microbiomics . Retrieved June 6, 2013, from The Economist:

The Economist. (2012, August 18). The human microbiome: Me, myself, us. Retrieved June 4, 2013, from The Economist:

The Economist. (2012, November 3). Treating disease with microbes: Bugs in the system. Retrieved June 4, 2013, from The Economist:

Flam, F. (2012, June 9). Retrieved June 7, 2013, from We and Our Microbes are in This Together:

Jane A. Foster, K.-A. M. (2013, May). Gut–brain axis: how the microbiome influences anxiety and depression . Retrieved June 4, 2013, from ScienceDirect:

Judson, O. (2009, July 21). Microbes 'R' Us. Retrieved June 7, 2013, from New York Times:

Perelman School of Medicine at the University of Pennsylvania. (2012, March 15). Genetic Variation in Human Gut Viruses Could be Raw Material for Inner Evolution, Perelman School of Medicine Study Finds. Retrieved June 13, 2013, from Penn Medicine:

Perry, W. (2012, July 6). Protective Skin Microbes Help Fight Off Disease,. Retrieved June 5, 2013, from LiveScience:

Siegfried, T. (2013, May 28). Microbes at home in your gut may also be influencing your brain. Retrieved June 4, 2013, from Science News:

Virginia Tech. (2013, February 8). Villain stomach bug may have a sweet side. Retrieved June 6, 2013, from EurekaAlert!:

Xu J, M. M. (2007, July 5). Evolution of symbiotic bacteria in the distal human intestine. Retrieved June 6, 2013, from

Zimmer, C. (2006, January 3). From Bacteria to Us: What Went Right When Humans Started to Evolve? Retrieved June 4, 2013, from New York Times:

Zimmer, C. (2013, May 22). Meet Your New Symbionts: Trillions of Viruses . Retrieved June 4, 2013, from National Geographic: