Gut reactions

In the not-too-distant future, predicted Steven Gill, a microbial geneticist who helped lead the study at the Institute for Genomic Research in Madison, US, doctors will test for subtle changes in the numbers and kinds of microbes in people’s guts as early indicators of disease. Doctors may prescribe live bacterial supplements to bring certain physiological measures back into normal range. And drug companies will invent compounds that mimic or amplify the actions of helpful bacteria. ‘‘These microbes are master physiological chemists,’’ said Jeffrey I. Gordon of Washington University in St Louis, another team member. ‘‘Understanding their biosynthetic capabilities and the pathways by which they operate could be the starting point for a 21st-century pharmacopoeia.’’

For all their numbers, bacteria account for only about three pounds of the average person’s weight. Just how important those three pounds are, however, has been difficult to appreciate until now.

The new study, described recently in the journal Science, took a novel approach. Rather than struggling to grow the body’s myriad microbes and testing their ability to perform various biochemical reactions, the team used tiny molecular probes to retrieve tens of thousands of snippets of bacterial DNA from smidgeons of the intestinal output of two volunteers. By comparing the DNA sequences of those snippets with those of previously studied bacteria, the team was able to sort many of the invisible bugs into known families. Hundreds of others, it became clear, belong to microbial families unknown to science until now.

The team members went further. By comparing the genetic puzzle pieces with similar sequences stored in databases, they were able to determine what biological functions many of these microbes perform in the gut. Some of the bacteria have the genetic machinery to make essential vitamins that are not found in the diet and that human cells can barely manufacture, including several B vitamins. Others make enzymes that can break the chemical bonds in plant fibres where a plant’s nutritional energy is stored.

Some bacteria in the gut break down flavonoids and other chemicals made by plants that could cause cancer or other illnesses if they were not neutralised in the intestines. Others have the genetic capacity to scavenge hydrogen gas from the gut—a byproduct of digestion that can kill helpful bacteria—and convert it into methane. That makes the intestines a more biologically friendly place, while contributing in sometimes embarrassing moments to Earth’s accumulation of greenhouse gases.

k Weiss

Five ways that intestinal microbes keep us alive, by doing what we can’t:

Breakdown of complex plant sugars to extract energy

Fermentation of dietary fibre

Conversion of hydrogen gas to methane

Breakdown of plant toxins that cause cancer and other diseases

Vitamin synthesis, including several B vitamins