Bacterial Inheritance Leads Researchers to Rethink Heredity

Female mice can pass down a bacterium to their offspring that causes them to have intestinal problems mimicking a genetic disease. Ho New / REUTERS

A child's genetics aren't just a product of mom and dad's DNA: Genes from mom's gut microbes also play a role in how the kid turns out.

Over the last decade or so it's become increasingly clear that the microbiota—all the microorganisms that live in the gastrointestinal tract and elsewhere in the body—play a huge role in health and development. But when we speak about somebody's genetics, we still usually mean their own DNA.

It may be time to change our thinking, says Dr. Thaddeus Stappenbeck, a researcher at Washington University in St. Louis. That's because microbes (and their genetic material, collectively known as the microbiome) within the body can influence all kinds of traits as disparate as digestion and behavior, and can be passed down to offspring, he says. Just like a person's own DNA.

As Stappenbeck and co-authors have shown in a paper published this week in the journal Nature, genes from bacteria within the gut of a mother mouse can be passed to her offspring, causing them to develop digestive problems that mimic a genetic disease.

The scientists actually came upon this finding by accident, while studying the genetic basis of inflammatory bowel diseases in mice. They knew that the digestive issues, characterized by inflamed intestines, can be caused by a genetic mutation. But what confused them was that they found that the diseased mice they were studying harbored no genetic mutations that could be connected to digestive problems.

So, the scientists created two groups of the mice. Both groups were genetically normal and they were almost identical except one population had intestinal problems. These mice had low levels of IgA, an antibody that defends the gut and other tissues from infection. The researchers then let the two groups mix. The intestinal problems spread throughout the population, and were passed on to offspring. (They were born after the mice were separated once again.) After digging into it, the researchers concluded that the mice with unusual intestines harbored a type of bacteria called Sutterella that appeared to be causing the changes.

The scientists identified this bacterium by sequencing genes within the microbe's ribosome, a cellular component where genes are made. They next did a series of tests to show that proteins coded by a Sutterella gene were actually degrading IgA within mice intestines.

It had previously been known in the scientific community that genetically identical animals could develop different behaviors and even diseases and pathologies when reared in different facilities. Researchers thought it was due to changes in the animals' microbiota, and this study shows one specific way that can happen, says Sarkis Mazmanian, a microbiologist at CalTech who was not involved in the current study.

"This has been a mystery for many of us: exactly how the microbiota are affecting behavior," Mazmanian says. "The advance here is an important one, showing that differences in microbiota can affect different arms of the immune system" and gastrointestinal function.