Your mother's DNA may have determined your eye color, but some traits that you thought came from her may instead have come from the DNA of bacteria she passed on to you soon after birth, a new study finds.
The study found that a mother mouse can pass along to her offspring a susceptibility to intestinal disorders, such as inflammatory bowel disease, by way of a gut-residing bacterium called Sutterella, the researchers reported in the journal Nature on Feb. 16.
Scientists have long speculated that a mother can transfer beneficial bacteria to her offspring through the birthing process and then through breast-feeding and kissing. These myriad bacteria species quickly spread and cover an infant's skin, mouth and digestive tract.
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This new finding, however, is the first identification of a specific trait that an offspring can inherit — in this case, a deficiency of a blood protein called immunoglobulin A, or IgA, which is the source of the bowel condition — caused by a specific bacterium that can be transferred from the mother to her offspring. IgA helps the body fight infection.
"The implications for mouse experiments are profound, and could help us cut through some persistent sources of confusion," in genetic research, said Dr. Thaddeus Stappenbeck, an immunologist at Washington University School of Medicine in St. Louis and a co-author of the new study.
Until now, most doctors have thought that IgA deficiency, seen in people with diseases such as chronic diarrhea, Crohn's disease and ulcerative colitis, is primarily hereditary, meaning the deficiency is inherited through one's genes. The new finding suggests that bacterial forces transmitted from mother to infant also are at play, perhaps to a significant extent.
The scientists, led by Stappenbeck and Herbert Virgin, also of Washington University, were studying the genetics of inflammatory bowel diseases. Their discovery of the bacterial interplay, they said, came about by accident.
A previous study in mice that suggested that genes played a role in IgA levels seen in Crohn's disease, Stappenbeck told Live Science. "However, over time, we could not repeat this result."
The scientists found, to their surprise, that their genetically modified mice had low levels of IgA. This was not a trait for which the mice were bred.
Through various experiments trying to isolate the problem, they discovered that adult mice passed Sutterella bacteria to each other via their feces and that mother mice spread the Sutterella directly to their offspring soon after birth, passing on this "trait" of low IgA. Sutterella inhibits the secretion of IgA.
The finding might help resolve a common bug in lab experiments, the researchers said. For years scientists have noticed differences between genetically modified mice — which, theoretically, should all be identical twins, but in reality can exhibit different traits from cage to cage and lab to lab — making it difficult for scientists to reproduce experimental results.
So, at a laboratory level, the new finding is important in understanding how to better design studies with "identical" genetically modified animals to take into account the variation that comes from bacteria passed from mother to offspring.
"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," Stappenbeck said.
The broader question is what it all means evolutionarily. Sutterella may be but one of many bacteria altering human susceptibility to disease, the scientists said.
Virgin, an immunologist, said he thinks the new findings will produce a more complicated but also much more insightful picture of how human, bacterial and viral genes influence human health.
"We may need to substantially expand our thinking about [bacterial and viral] contributions, and perhaps the contributions of other microorganisms, to genetics and heredity," Virgin said.
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