Identical twins may not be so identical when it comes to gut bacteria

Identical twins may share appearances, mannerisms, even clothes — but the microbes living in their guts are anything but the same. By comprehensively sequencing microbial genes in the gut, researchers have found that communities of bacteria in adult identical twins differ dramatically. The findings, to appear online during the week of March 29 in the Proceedings of the National Academy of Sciences, give scientists a deeper understanding of what makes one person’s intestinal bacteria different from another’s.

Figuring out what’s behind the composition and function of a person’s gut bacteria is “a very important problem,” comments microbiologist Frederic Bushman of the University of Pennsylvania School of Medicine in Philadelphia. Bacteria in the intestines spur digestion, manufacture vitamins and keep people healthy; changes in gut bacteria have been linked to irritable bowel syndrome and metabolic disorders. Because of their role in breaking down and absorbing the nutrients from food, gut bacteria may even influence a person’s weight.

In the new study, the researchers comprehensively sequenced bacterial genes in stool samples from a pair of identical twins, 26-year-old obese women from Missouri. Since identical twins have nearly identical DNA, any differences between their gut microbial populations would have to be due to non-genetic factors such as diet, disease history or exposure to antibiotics.

“Diet is probably a huge, huge factor,” says study coauthor Jeffrey Gordon, a systems biologist at Washington University School of Medicine in St. Louis, although researchers still don’t know exactly what factors shape bacterial variation in the gut.

The twins shared in their gut microbes only about 17 percent of DNA sequences, based on a classification scheme developed by the researcher. What’s more, about 64 percent of the gene groups identified had not been seen in previous studies.

The results “change our sense of personal identity,” says Gordon. “We really do have different constellations of species.”

One of the notable differences between the twins was a gene group that produces a protein family called dockerins, which form assemblies that microbes use to break down cellulose. One twin’s intestinal bacteria possessed these dockerin genes, but the other’s lacked them.

In addition to looking for which genes were present in the gut microbiomes, the team went a step further and determined which ones were active. Some microbial genes found in both twins showed similar levels of expression, but the researchers found differences, too.

“This is the first look at the repertoire of expressed genes,” Gordon says, and the results will serve as a baseline for understanding how gut microbe genes function differently in different people.