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.