The Good With the Bad

The diversity of species that make up the microbiome is hard to fathom. But it is even more difficult to understand how the immune system copes with this onslaught. In any one person’s mouth, for example, the scientists of the Human Microbiome Project found about 75 to 100 species. Some that predominate in one person’s mouth may be rare in another person’s. Still, the rate at which they are being discovered indicates that there may be as many as 5,000 species of bacteria that live in the human mouth.

“The closer you look, the more you find,” said Susan M. Huse of the Marine Biological Laboratory in Woods Hole, Mass., a contributor to the microbiome project.

Although the project has focused largely on bacteria, the microbiome’s diversity is wider. For example, our bodies also host viruses.

Many species in the human “virome” specialize in infecting our resident bacteria. But in the DNA samples stored in the Human Microbiome Project’s database, Kristine Wylie of Washington University and her colleagues are finding a wealth of viruses that target human cells. It is normal, it seems, for people to have a variety of viruses busily infecting their human hosts. “It’s really pretty striking that even in these healthy people, there really is a virome,” Dr. Wylie said.

The microbiome also includes fungi. In the June 8 issue of the journal Science, David Underhill, a research scientist at Cedars-Sinai hospital in Los Angeles, and his colleagues reported on a wealth of fungal species in the guts of humans and other mammals. In mice, for example, they cataloged 100 species of fungi that are new to science, along with 100 already known. This diversity is all the more remarkable when you consider that it is tolerated by an immune system that has evolved to fight off microbes. Scientists have only a dim understanding of how the system decides which to kill and which to tolerate.

Immune cells fight fungal infections, for example, with a protein called dectin-1, which attaches only to fungi. But Dr. Underhill and his colleagues found that dectin-1 is also essential for tolerating harmless fungi. When they engineered mice that couldn’t produce dectin-1, the mice responded to harmless fungi by producing so much inflammation that their own tissues were damaged.

It’s a good thing that the immune system can rein itself in, because the microbiome carries out many services for us. In the gut, microbes synthesize vitamins and break down tough plant compounds into digestible bits.

Skin bacteria are also essential, Dr. Segre said. “One of the most important functions of the skin is to serve as a barrier,” she said. Bacteria feed on the waxy secretions of skin cells, and then produce a moisturizing film that keeps our skin supple and prevents cracks — thus keeping out invading pathogens.

Дата добавления: 2015-11-14