参与人体微生物项目（Human Microbiome Project）的研究者已经记录了1万多种人身上的微生物。这是一个国际性项目，由美国国立卫生研究院（U.S. National Institutes of Health）牵头。人的肠道内平均有大约38万亿个微生物，重量约0.2公斤。和欠发达国家的居民相比，工业化国家居民所携带微生物的多样性要低得多，人们认为首要原因是抗生素在医药和农业两个领域中的广泛使用。其他原因还包括自然分娩率和母乳喂养率较低——自然分娩和母乳喂养可将母亲的微生物传递给孩子，这也许能解释为什么剖腹产婴儿的哮喘和过敏发病率较高。只喝配方奶粉的婴儿患肥胖症和糖尿病的风险较高。社交、户外活动以及和动物的接触较少可能也会减少这些微生物的数量。多年来，科学家一直都知道和宠物犬一起长大会降低儿童患哮喘的可能性——一项研究称降幅可达13%，而且主要理由就是微生物多样性。看来抚摸爱犬以及被它舔舐可以让家庭成员进行微生物交换。
Even when you think you’re alone, you’ve got a lot of company. Of the cells you carry around, only about half, it turns out, are human. The rest belong to micro-organisms — bacteria, fungi, protozoans and viruses — that are on and inside you. That may sound unsanitary, but most of them are either harmless or beneficial. Many researchers think that in our overzealousness to kill bad germs, we’ve knocked off some underappreciated good ones, depleting the diversity of our microbiota, whose collective genes are known as the human microbiome. According to one theory, this has contributed to the recent increase, in industrialized countries, of autoimmune diseases such as diabetes, multiple sclerosis and lupus. That’s led to a spate of new research on whether a better relationship with our fellow travelers could undo that rise, and whether some of our microbes could be deployed to make us even healthier.
Scientific findings have come quickly, connecting human microbiota to infectious diseases such as HIV and hepatitis, non-communicable ones like heart disease and cancer, and even psychiatric disorders such as autism and depression. The discoveries have sparked an explosion of research by drug companies and academic labs into possible treatments and cures. The idea, usually, is to alter organisms in the gut, where about 99 percent of human microbiota live. Some of the most promising research involves fecal transplantation — the introduction through colonoscopy, endoscopy or enema of a donor stool. In early human trials, it’s shown potential as a superior treatment for recurrent infection with C. difficile, a pathogen often contracted in hospitals that causes diarrhea and kills about 30,000 people a year in the U.S. Obesity has been discussed as another tantalizing target. In experiments, fat mice transplanted with microbes from thin mice lost weight and vice versa.
Researchers connected to the Human Microbiome Project, an international effort led by the U.S. National Institutes of Health, have catalogued more than 10,000 microbial species among our companion germs. The average human gut contains about 38 trillion microbes, weighing about 0.2 kilograms (0.4 pounds). Compared with people in less-developed countries, those in industrialized states have significantly less diversity in the microorganisms that populate their bodies. An excessive use of antibiotics, both in medicine and agriculture, is thought to be a leading cause. Lower rates of natural childbirth and breast feeding are others: Those practices pass microbes from mother to child, perhaps explaining why cesarean section babies have higher rates of asthma and allergies, and formula-fed babies are at an elevated risk of obesity and diabetes. Less socializing, outdoor activity and exposure to animals may also deplete microbiota. Scientists for years have known that growing up with a dog lowers a child’s chance of having asthma — by 13 percent, according to one study — and microbial diversity has become the leading explanation. Family members, it seems, swap microbes by petting their dogs and being licked by them.
Researchers stress that therapies based on microbiome science will take time to develop. Meanwhile, there’s widespread agreement among scientists that people in industrialized countries would do themselves a favor by going easy on antibiotics. Already blamed for breeding resistance in bacteria and thus creating superbugs, antibiotic overuse is now a suspect in killing off good as well as bad bacteria in the human gut. Physicians say they often prescribe unnecessary antibiotics for viral infections because their patients expect it and there’s no time to explain that a bacteria killer won’t destroy a virus. A public-information campaign in the U.S. aimed at reducing patient demand has corresponded with a decline in antibiotic prescriptions. U.S. farmers use antibiotics intensively on livestock both to treat and prevent disease. Usage is much lower in most countries in the European Union, which has banned dosing healthy animals since 2006. Another possibility for encouraging a better microbial balance is to take in “good” microbes, called probiotics. They can be found in food such as yogurt, miso and soy beverages or in dietary supplements. Some researchers see potential in probiotics, but the case for them requires more investigation, and there are no established protocols for how much or what strains to consume for various purposes.