Tending the Body’s Microbial Garden

By CARL ZIMMER

Published: June 18, 2012

For a century, doctors have waged war against bacteria, using antibiotics as their weapons. But that relationship is changing as scientists become more familiar with the 100 trillion microbes that call us home — collectively known as the microbiome.

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“I would like to lose the language of warfare,” said Julie Segre, a senior investigator at the National Human Genome Research Institute. “It does a disservice to all the bacteria that have co-evolved with us and are maintaining the health of our bodies.”

This new approach to health is known as medical ecology. Rather than conducting indiscriminate slaughter, Dr. Segre and like-minded scientists want to be microbial wildlife managers.

No one wants to abandon antibiotics outright. But by nurturing the invisible ecosystem in and on our bodies, doctors may be able to find other ways to fight infectious diseases, and with less harmful side effects. Tending the microbiome may also help in the treatment of disorders that may not seem to have anything to do with bacteria, including obesity and diabetes.

“I cannot wait for this to become a big area of science,” said Michael A. Fischbach, a microbiologist at the University of California, San Francisco, and an author of a medical ecology manifesto published this month in the journal Science Translational Medicine.

Judging from a flood of recent findings about our inner ecosystem, that appears to be happening. Last week, Dr. Segre and about 200 other scientists published the most ambitious survey of the human microbiome yet. Known as the Human Microbiome Project, it is based on examinations of 242 healthy people tracked over two years. The scientists sequenced the genetic material of bacteria recovered from 15 or more sites on their subjects’ bodies, recovering more than five million genes.

The project and other studies like it are revealing some of the ways in which our invisible residents shape our lives, from birth to death.

A number of recent reports shed light on how mothers promote the health of their children by shaping their microbiomes. In a study published last week in the journal PLoS One, Dr. Kjersti Aagaard-Tillery, an obstetrician at Baylor College of Medicine, and her colleagues described the vaginal microbiome in pregnant women. Before she started the study, Dr. Aagaard-Tillery expected this microbiome to be no different from that of women who weren’t pregnant.

“In fact, what we found is the exact opposite,” she said.

Early in the first trimester of pregnancy, she found, the diversity of vaginal bacteria changes significantly. Abundant species become rare, and vice versa.

One of the dominant species in the vagina of a pregnant woman, it turns out, is Lactobacillus johnsonii. It is usually found in the gut, where it produces enzymes that digest milk. It’s an odd species to find proliferating in the vagina, to say the least. Dr. Aagaard-Tillery speculates that changing conditions in the vagina encourage the bacteria to grow. During delivery, a baby will be coated by Lactobacillus johnsonii and ingest some of it. Dr. Aagaard-Tillery suggests that this inoculation prepares the infant to digest breast milk.

The baby’s microbiome continues to grow during breast-feeding. In a study of 16 lactating women published last year, Katherine M. Hunt of the University of Idaho and her colleagues reported that the women’s milk had up to 600 species of bacteria, as well as sugars called oligosaccharides that babies cannot digest. The sugars serve to nourish certain beneficial gut bacteria in the infants, the scientists said. The more the good bacteria thrive, the harder it is for harmful species to gain a foothold.

As the child grows and the microbiome becomes more ecologically complex, it also tutors the immune system. Ecological disruptions can halt this education. In March, Dr. Richard S. Blumberg of Harvard and his colleagues reported an experiment that demonstrates how important this education is.

The scientists reared mice that lacked any microbiome. In their guts and lungs, the germ-free mice developed abnormally high levels of immune cells called invariant natural killer T cells. Normally, these cells trigger a swift response from the immune system against viruses and other pathogens. In Dr. Blumberg’s microbe-free mice, however, they caused harmful inflammation. As adults, the mice were more likely to suffer from asthma and inflammatory bowel disease.

This experiment parallels studies of children in recent years. Children who take high levels of antibiotics may be at greater risk of developing allergies and asthma later on, many researchers have suggested.

Dr. Blumberg and his colleagues found that they could prevent the mice from becoming ill by giving them bacteria while they were still young. Acquiring a microbiome as an adult did not help the rodents.

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.