Like pop star Britney Spears, microbes have been getting a lot of bad publicity lately. From methicillin-resistant Staphylococcus aureus (MRSA) outbreaks to E. coli in our food, the headlines have made it abundantly clear that bacteria are very inventive when it comes to harming human health. With all of this bad news, we often forget that the vast majority of microbes are harmless. In fact, many microbes benefit other organisms. From the bacteria living inside the rumens of cows, to bacteria in the roots of soybean plants, many organisms depend on bacteria living symbiotically with them for growth and survival. But these microbial symbioses are not just found in farm fields. Not far from the computer screen where you are reading this article, you are hosting a beneficial microbial community in your intestine that can have substantial impacts on your health.

The number of microbes living inside our intestines is enormous. In the average human intestine, there are up to 100 trillion microbes, more than 10 times the number of human cells in the entire body! Scientists are still discovering new species of intestinal microbes since many of these organisms are very difficult to grow in the lab. Using DNA based techniques, more than 500 species of bacteria have been found to live in the human intestine. People with similar diets and lifestyles often have relatively similar microbial communities in their bodies, but the exact identities of the microbes in your intestine can be as unique as your fingerprint.

Gut Microbes and Human Health

Just exactly what these microbial communities in our intestines do is still under investigation by many different labs, but several metabolic and protective functions have been identified. These include curtailing the growth of harmful microbes, regulation of the immune system, production of vitamins, and the break down of complex compounds that our own digestive systems cannot digest.

These seemingly small processes carried out by our unseen gut tenants may have large effects on human health. Several recent studies have shown that intestinal microbes can regulate the amount of fat storage in mice suggesting that the microbes within a person’s intestine can affect their propensity for weight gain. In one study, scientists actually took the intestinal microbial community of obese people and added them to microbe free mice. Interestingly, these mice experienced greater weight gain than mice receiving microbes from non-obese people.

Links have also been made between intestinal microbes and the development of cancer. For example, when microbiologists studied the microbial communities of people at risk for colon cancer, they found that certain types of bacteria are associated with individuals at high risk compared with individuals at low risk. While these studies suggest that intestinal microbial communities play a role in shaping human health, the results are still preliminary because they cannot tell us just how important intestinal microbes are compared with other factors that affect health, such as genetics and lifestyle.

Microbes from Mom?

With such a large and complex community of bacteria living in the human intestine, scientists have been trying to determine the factors that affect which microbes live in your particular intestine. Factors such as diet, the use of antibiotics, and lifestyle, have been proposed as major controllers of the diversity of gut microbes. The microbial communities of the people around you may also affect what microbes live in your intestine. One study showed that the microbial communities from individuals within families are more similar than they are between families. However, with these types of studies it is difficult to understand how much of a role the genetic relatedness of people plays in shaping their intestinal bacteria compared to living in a common environment and eating similar foods.

To examine the potential for the spread of beneficial microbes from parents to children in more detail, one recent study followed the development of intestinal microbial communities in 14 healthy babies. By analyzing bacterial DNA sequences in stool samples, the scientists tracked how many different species of bacteria were present right after birth until the infants were one year old. They also collected samples from potential bacterial sources in each baby’s environment, including breast milk and stool samples from family members. In most cases, there was a large amount of variation in the groups of bacteria that initially colonized the infants’ intestines, but as the infants aged, they began to develop similar intestinal communities that resembled intestinal communities of adults. Some bacterial groups matched those present in the mother’s milk and the stool of family members suggesting that family members were a source of bacteria, but many bacteria also came from other unidentified sources in the environment.

Managing Microbes

As our knowledge of the role of beneficial microbes in human health becomes more advanced, there is a growing interest in trying to integrate this knowledge into human healthcare. The most apparent application of this growing discipline is in the use of probiotics, dietary supplements that contain bacteria or fungi. These supplements are intended to re-establish beneficial microbes in a digestive system that may have been altered by stress or consumption of medications, such as antibiotics. Unfortunately, the production of many of these products is unregulated and the efficiency of most of these treatments remains unknown. Some initial research with people who suffer from Chron’s Disease, a disorder associated with inflammation of the digestive tract, suggests that probiotics may help alleviate symptoms, but just how much we can manipulate our intestinal microbes to affect our health remains to be seen.

–Benjamin E. Wolfe, Organismic and Evolutionary Biology, Harvard University

For More Information:

Gross, L. 2007. Microbes Colonize a Baby’s Gut with Distinction. PLoS Biol 5(7): e191:
< http://biology.plosjournals.org/perlserv/?request=get-document&doi=10.1371%2Fjournal.pbio.0050191 >

Primary Literature:

Dethlefsen, L., M. McFall-Ngai, and D.A. Relman. 2007. An ecological and evolutionary perspective on human-microbe mutualism and disease. Nature 449 (18): 811-818.

Gaurner, F., and J-R Malagelada. 2003. Gut flora in health and disease. The Lancet 361: 512-519.

F Backhed, H. Ding, T. Wang, L. V. Hooper, G. Y. Koh, A. Nagy, C. F. Semenkovich, and J. I. Gordon. 2004. The gut microbiota as an environmental factor that regulates fat storage. Proceedings of the National Academy of Sciences of the United States of America 101 (44): 15718-15723.

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