The Importance of “The Little Guys:” Our Gut Microbiome

It’s easy to take for granted the many beneficial microbes that live on, or inside of, our bodies. We don’t usually think about them unless something goes wrong, and, honestly, we don’t know a whole lot about them. But we’re learning. And as our understanding improves, we’re increasingly finding that these bacteria, yeast, and other microbes – collectively referred to as the human microbiome — likely serve very important functions. For example, it’s been recently found that the gut microbiome (the hundreds of species of microbes living in our gastrointestinal tract) is not only related to a person’s age and where they live, but it’s also linked to obesity, autoimmune diseases, infections, and other medical conditions.

To piece together the puzzle of gut microbiome functionality, researchers compare the microbes found in fecal samples from different people. This is how it was found, for example, that adults have a different set of gut microbes than infants, but they start gaining the adult set when they around a year old. Last year, an in-depth study found that there’s also a large amount of variation between the gut microbiomes of people living in different parts of the world – specifically, people in a United States metropolis had less diverse gut microbiomes than people in rural Africa and South America. What’s ironic about this is that people in the U.S. probably have greater diversity in the food they eat. The lead author suggests that the lack of microbiome diversity may be due to lifestyle, hygiene, and (over)use of antibiotics. By better understanding some of these differences, we may be able to better treat related medical conditions, of which there are likely many.

Obesity is one condition that researchers have been repeatedly exploring via the gut microbiome – they’ve found that, basically, obese individuals have a less diverse microbiome than “lean” individuals. But what’s more interesting is studies in mice revealed that when healthy mice received microbe transplants from obese humans, the mice did not become obese when they were co-housed with lean mice. This is because mice eat each other’s feces, and the less-diverse microbiome from obese individuals has niches that can be filled by the more-diverse lean microbiome. But when the mice with microbiomes from obese individuals were alone or fed high-fat diets, they gained weight – the authors suggest this is because the bacteria that best colonize lean individuals cannot get the food they need from the unhealthy diet.

Looking at these studies, it becomes apparent that it’s difficult to differentiate between causation and correlation. Just one more example along these lines is a link between the gut microbiome and autoimmunity, specifically rheumatoid arthritis. Clearly much remains to be discovered.

We are only beginning to apply some of our limited knowledge of the gut microbiome in useful ways, such as fecal transplantations and pills to fight Clostridium difficile infections, which can cause severe diarrhea and result in hospitalization. In September of this year, C. difficile was identified by the Centers for Disease Control and Prevention as one of the most damaging types of antibiotic-resistant bacteria. But fecal transplants have been found to be very successful at treating these infections.

It will be fascinating to watch the burgeoning microbiome field grow over time and, undoubtedly, develop novel treatments for medical conditions to overall improve our quality of life.

For further reading:

• Chana Palmer et al.’s article “Development of the Human Infant Intestinal Microbiota” in PLOS Biology
• Tanya Yatsunenko et al.’s article “Human gut microbiome viewed across age and geography” in Nature
• Ruth Williams’s article “What Bugs Are in Your Gut?” in The Scientist
• Teisha J. Rowland’s article “Fighting Antibiotic Resistance”
• Ed Yong’s article “Obesity via Microbe Transplant” in The Scientist
• Abby Olena’s article “Gut Microbes May Impact Autoimmunity” in The Scientist
• Elizabeth Pennisi’s article “Using Gut Bacteria to Fight Diarrhea” in Science
• Abby Olena’s article “Fecal Pills” in The Scientist
• Tracy Vence’s article “Tracking Fecal Transplants” in The Scientist
• Debamita Chatterjee’s article “Eat Less and Live Longer? in The Scientist
• ScienceDaily’s article “Microbes Can Influence Evolution of Their Hosts”
• Teisha J. Rowland’s book Biology Bytes: Digestible Essays on Stem Cells and Modern Medicine