Traits are passed down to offspring through the DNA of the parents -- that's basic biology. But the discovery that DNA in bacteria living in a parent's body can also send a trait down to offspring is about to complicate everything.

The finding means researchers need to consider a significant new factor, the DNA of microbes passing from mother to offspring, in their efforts to understand genetic influences on health and illness, scientists at the Washington University School of Medicine in St. Louis say.

"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 Herbert W. Virgin IV. "But our results show bacteria stepping over the line.

"This suggests we may need to substantially expand our thinking about their contributions, and perhaps the contributions of other microorganisms, to genetics and heredity."

Most bacteria in our bodies are commensal, meaning they cause no cause harm and in fact often confer beneficial consequences, the researchers explain.

Commensal bacteria can influence traits such as behavior and weight, but it has always been believed bacteria exerting such effects were acquired during a person's life.

The new study is the first to suggest bacterial DNA may pass from parent to child to affect specific traits such as immunity and inflammation.

The finding could help explain a significant problem encountered in lab studies using genetically engineered mice, the researchers say.

In a number of research fields, scientists have seen their experiments suddenly veer off-track with the sudden, unexplained appearance of a new or altered trait in their subject mice.

While these unexpected traits can spread from one mouse habitat to another, suggesting a contagious microbial infection, they also have been seen passing from mothers to offspring, suggesting a genetic basis.

The Washington University researchers, Virgin and Thaddeus Stappenbeck, encountered such problems in their own research into inflammatory bowel diseases, leading them to try and understand the cause and source of the unexpected traits.

They found a bacterium that explained both methods of the trait appearing; mice housed together could be affected by a normal spreading of the bacteria, while mouse mothers could also pass it to their offspring.

That second method has brought about a major change in thinking, as it suggests traits affected by bacteria can pass down from mothers to offspring in the same way as traits affected by DNA, the researchers say.

"The implications for mouse experiments are profound and could help us cut through some persistent sources of confusion," Stappenbeck says. "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."

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