Although it is well known that genetic traits are passed from parent to child through DNA, a new study published in Nature finds that traits can also be passed to the child via the DNA of bacteria in the mothers’ body.

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The new study shows how genetic traits in mice can be affected by certain bacteria and how the DNA of these bacteria can be passed from mother to child.

“We have kept bacteria on one side of a line separating the factors that shape our development – the environmental side of that line, not the genetic side,” says co-senior author Dr. Herbert W. Virgin IV, the Edward Mallinckrodt professor of Pathology and head of the Department of Pathology and Immunology at Washington University School of Medicine in St. Louis (WUSTL).

“But our results show bacteria stepping over the line,” Dr. Virgin continues. “This suggests we may need to substantially expand our thinking about their contributions, and perhaps the contributions of other micro-organisms, to genetics and heredity.”

“Commensal bacteria” are organisms that live within our bodies but do not cause disease. However, these bacteria are known to influence weight, behavior and other traits. Scientists had previously believed the genetic traits associated with commensal bacteria to be acquired during a person’s lifetime rather than inherited.

However, the new study shows for the first time that bacterial DNA can pass between a mother and her offspring, and that the transference of this DNA influences traits including immunity and inflammation.

In the new study, the researchers found that mice with certain bacteria inherited from their parents were more susceptible to a gut injury caused by chemical exposure. Mice carrying other inherited bacteria, however, were less susceptible to this condition.

The researchers believe that the findings of the study – as well as expanding knowledge on how DNA is transmitted – also have important practical implications for science.

Across several fields of research, scientists have reported an occasional glitch when conducting experiments using genetically engineered mice, where new or altered genetic traits can suddenly appear in the mice in a way that cannot be easily explained.

Scientists believed that microbial infection was the most likely culprit, as these traits appeared to spread between mouse habitats. However, what confused scientists is that the traits were also consistently passed from mother to offspring, implying a genetic cause.

The findings of the new study suggest an explanation for this by showing how traits in mice can be affected by bacteria that is spread normally among a cohabiting population, but also how those bacteria-influenced traits are passed from mothers to offspring.

Dr. Thaddeus Stappenbeck, a professor of pathology and immunology at WUSTL, says of the study:

The implications for mouse experiments are profound and could help us cut through some persistent sources of confusion. When we study mice, we have to account for the possibility that inherited bacteria and their genes could be influencing the trait we’re trying to learn about.”

Dr. Stappenbeck adds that a workaround for the problem of altered or new traits in genetically modified mice could be to house experimental and control mice in the same colony, which would ensure that inherited microbes are present in both groups.

Dr. Virgin says he expects a much more complicated picture of heredity to emerge following the findings of this study that will provide more insights of how human, bacterial and viral genes influence human health.