We are vastly, ridiculously, hopelessly, humblingly outnumbered: For every one human cell, there are an estimated 10 single-cell microbes in or on us - at least 100 trillion in all - nestled in our guts and in our urogenital tracts, lying on our skin and happily ensconced in our mouths and noses - entire civilizations of fungi and protozoa and (mostly) bacteria that eat and breathe, evolve, reproduce and die.
Before you reach in horror for the hand sanitizer or industrial strength mouthwash, you might want to keep something in mind. If we disrupt our "human microbiota," we do so at our peril.
In an effort to obliterate disease-causing microbes through antibiotics and other anti-microbials - from the pills prescribed to the meat we eat to the hand-sanitizer dispensers everywhere you look - we are carpet-bombing our microbiota. And that war on germs has taken a toll on beneficial bugs, too.
For example, the bacterium Helicobacter pylori, which causes ulcers and is linked to stomach cancers, was once in almost everyone's gut. It is now found in just 6 percent of U.S. children, Science magazine reported in 2011, probably due to the widespread use of anti-microbials.
That should mean fewer ulcers, but there's a dark lining to that silver cloud: H. pylori may ward off asthma..
It isn't just asthma. The list of illnesses linked to the population of microbes in our bodies seems to get longer every month, but here's a quick tour.
- Chronic sinusitis: Scientists led by Susan Lynch of the University of California, San Francisco, reported the nasal microbiota of chronic rhinosinusitis (CRS) patients had low levels of good-for-you bacteria and abnormally high levels of not-so-good bacteria. When the scientists replicated the microbiota of CRS patients in mice, the animals developed sinusitis. Transplanting the good bug into mice inhibited the growth of sinusitis-causing bacteria and prevented infection.
- Excessive appetite: H. pylori - of ulcer-causing fame - regulate the stomach's production of ghrelin, an appetite-stimulating hormone. Several labs have found that people whose stomachs harbor more H. pylori have less ghrelin and thus less hunger; conversely, fewer H. pylori means more ghrelin and greater likelihood of overeating.
- Autoimmune diseases: The microbes in your gut play a role in regulating your immune system. That means they also affect autoimmune diseases such as rheumatoid arthritis and Crohn's, researchers at the Mayo Clinic and elsewhere have found. One powerful gut bacterium has anti-inflammatory properties, protecting against recurrence of Crohn's disease. More generally, the immune system seems to consider some gut microbes dangerous invaders, unleashing an attack that produces the inflammation characteristic of arthritis and Crohn's.
- Atherosclerosis: Patients with this disease characterized by hardening of the arteries had different types of gut bacteria than healthy people, scientists reported in Nature Communications last December. In particular, atherosclerosis patients' gut bacteria had fewer genes for the production of natural antioxidants linked to heart health; patients therefore had less of these antioxidants in their blood.
- Obesity: The field of microbiota and health took off in 2006, when scientists led by Jeffrey Gordon of Washington University in St. Louis transferred gut bacteria from obese mice into thin ones. The thin mice ate no more than they used to, but they quickly started gaining weight. "Some microbes change how efficiently we metabolize food," explains biologist Rob Knight of the University of Colorado, who studies the genetics of the microbiota, called the microbiome.
A LOT OF GENES. A LOT.
The National Institutes of Health launched the Human Microbiome Project to study the role the microbiome plays in health and disease. Based on samples from 242 healthy 18- to 40-year-olds, the scientists announced the microbiome consists of some 8 million functional genes, or 360 times as many genes as those in our own DNA. "It's like a second genome," says Colorado's Knight, one of the project scientists.
Where all this leaves someone who wants to cultivate healthy microbiota is only starting to become clear. Until scientists can give us the recipe for healthy microbiota, experts say it pays to heed the advice of mainstream medical groups, which recommend restricting the use of antibiotics to only must-have circumstances, not every cough and sniffle. Consider buying antibiotic-free meat.
Be wary of manufacturers making exaggerated claims about probiotics and prebiotics. While some claims are supported by research, others "greatly outstrip the scientific evidence," says Knight.
But just because commerce has galloped ahead of science doesn't mean there is no scientific basis for manipulating the microbiome to improve health. "We can't change our first genome, the one we inherit from our parents," says Knight. "But we can change the second, the microbiome. And that holds real promise."