The Good, Bad and Ugly
Microscopic creatures—including bacteria, fungi and viruses— can make you ill. But what you may not realize is that trillions of microbes are living in and on your body right now. Most don’t harm you at all. In fact, they help you digest food, protect against infection and even maintain your reproductive health. We tend to focus on destroying bad microbes. But taking care of good ones may be even more important. You might be surprised to learn that your microbes actually outnumber your own cells by 10 to 1. “The current estimate is that humans have 10 trillion human cells and about 100 trillion bacterial cells,” says Dr. Martin J. Blaser at the New York University School of Medicine. New techniques allow scientists to study these rich microbial communities and their genes—the “microbiome.” In 2007, NIH launched the Human Microbiome Project to study microbes in and on the body. Earlier this year, researchers from almost 80 institutions published a landmark series of reports. They found that more than 10,000 different species occupy the human body. The microbiome actually provides more genes that contribute to human survival than the human genome itself (8 million vs. 22,000). Humans need bacteria and their genes more than most of us thought. One of the most important things microbes do for us is to help with digestion. The mix of microbes in your gut can affect how well you use and store energy from food. In laboratory experiments, transferring bacteria from certain obese mice to normal ones led to increased fat in the normal mice. Blaser and his colleagues are concerned that changes in our microbiome early in life may contribute to weight problems later. “We’re in the middle of an epidemic of obesity that is very severe,” Blaser says. “It’s relatively recent, it’s widespread across the United States and across the world, and increased calories and decreased exercise seem insufficient to explain this.” We might be changing our microbiome for the worse, he says, by using antibiotics too often. In a recent NIH-funded study, Blaser’s team found that lowdose antibiotic therapy affected the gut microbiomes of young mice. Antibiotics also altered how the mice used sugars and fats. After 7 weeks, treated mice had up to 15% more fat than untreated mice. This and other studies suggest that gut bacteria can affect both appetite and how you use energy in food. In related work, Dr. Leonardo Trasande, Blaser and colleagues analyzed data from more than 11,000 children. Although the results weren’t conclusive, they suggest that infants given antibiotics might be at increased risk of becoming overweight. More work will be needed to confirm this connection. “Microbes in our intestines may play critical roles in how we absorb calories,” Trasande says. “Exposure to antibiotics, especially early in life, may kill off healthy bacteria that influence how we absorb nutrients into our bodies, and would otherwise keep us lean.” Microbes are also important for your skin, one of the body’s first lines of defense against illness and injury. Skin health depends on the delicate balance between your own cells and the microbes that live on its surface.