Before exploring how else the microbiota influence our health, it’s worth taking a step back to cover some of the basics. It starts with birth. The bacteria and other microbes you enter the world with come from your mother’s skin (if you were a Cesarean baby) or birth canal. “At first, all your biota resemble your mother’s,” says Knight. “We don’t really know how fast the microbiota of the skin or gut or nose change, but within two or three years they’re like an adult’s,” having picked up microbes from the air, water, clothing, and diet. This last part is vital: What we eat shapes what microbes our guts harbor. In 2010, a study found that Japanese people harbor gut bacteria that digest nori, the seaweed used in sushi. Westerners don’t. Why? The Japanese consume lots of fish, which contain marine bacteria, which digest nori. “What you eat,” says microbiologist Justin Sonnenburg of Stanford University School of Medicine, is “one of the major determinants” of your gut microbiota.
It isn’t just obesity. The list of illnesses linked to the population of microbes in your body seems to get longer every month, but here’s a quick tour.
• Tooth decay. A single species of bacteria is the chief cause of most cavities: It turns the sugar we eat into acid that eats away tooth enamel. Scientists are working on a mouthwash that would kill this bacteria, raising hope that tooth decay will become a thing of the past.
• Chronic sinusitis. Last September, scientists led by Susan Lynch of the University of California, San Francisco, reported that the nasal microbiota of chronic rhinosinusitis (CRS) patients had low levels of good-for-you bacteria and abnormally high levels of a not-so-good bacteria. When the scientists depleted the microbiota of mice to make them like CRS patients’, sure enough, the animals developed sinusitis. And transplanting that bug into mice inhibited the growth of sinusitis-causing bacteria and prevented infection.
• Excessive appetite. Certain gut bacteria produce a compound called PYY, which makes you feel full and reduces how much you eat. Absent those bacteria, your brain doesn’t get the “stop eating” signal. And H. pylori (of ulcer-causing fame) regulates 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 the immune system and therefore autoimmune diseases such as rheumatoid arthritis and Crohn’s, researchers at the Mayo Clinic and elsewhere have found. For instance, 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, the gut bacteria of those with atherosclerosis had fewer genes for the production of natural antioxidants linked to heart health; patients therefore had less of these antioxidants in their blood.
In December 2007 the National Institutes of Health launched the Human Microbiome Project to study the role the microbiome—the genes in our microbiota—plays in health and disease. Based on samples from 242 healthy people, 18 to 40, Project scientists announced in 2012, the microbiome consists of some 8 million functional genes from about 10,000 individual species—or 360 times as many genes as the 22,000 or so in our own DNA. “It’s like a second genome,” said 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. One obstacle is that “nobody knows what an ideal or even normal human microbiome is,” says Jeff Leach, an anthropologist. To help find out, in 2012 he launched “American Gut.” A crowdsourced research project, it invites anyone to request a sample-collecting kit ($99), basically a super-long Q-tip that you use to swab used toilet paper and send back for analysis. A questionnaire asks about some of your general habits and lifestyle: what pets you have, whether you have taken antibiotics recently, what you ate over the past few days, and other behaviors thought to affect the gut microbiota. In return, you get back the list of your gut microbes and how you cluster or compare to other people.
That’s only the start. 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. “The scorched earth outcome of many broad-spectrum antibiotics is analogous to spraying poison all over your backyard plants and grass, and waiting to see what grows back,” says Leach. In both cases “invasive and maybe not-so-good species” can move in, with harmful results.
Be wary of manufacturers making exaggerated claims about probiotics and prebiotics. While some claims are supported by research [see “Post Investigates: Probiotics,” Jul/Aug 2012], 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 Colorado’s Knight. “But we can change the second, the microbiome. And that holds real promise.”
To view a video on the topic, go to saturdayeveningpost.com/microbiome.