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Listeria is a common food-borne bacterium that may be a greater risk for miscarriage in early pregnancy than thought.
According to researchers at the University of Wisconsin-Madison (UW-Madison) School of Veterinary Medicine who study how pathogens affect fetal development, the bacterium affects those with stressed immune systems and can change the outcome of pregnancy.
Golos and his collaborators published their results Feb. 21, 2017 in the journal mBio.
"The problem with this organism is not a huge number of cases. It's that when it is identified, it's associated with severe outcomes," says Charles Czuprynski, a UW-Madison professor of pathobiological sciences and director of the UW-Madison Food Research Institute.
Pregnant women are warned to avoid many of the foods — among them unpasteurized milk and soft cheese, raw sprouts, melon and deli meats not carefully handled — that can harbor listeria, as the bacterium is known to cause miscarriage and stillbirth, and premature labor. These severe outcomes demand a zero-tolerance regulatory policy for listeria in ready-to-eat foods.
"It's striking that mom doesn't get particularly ill from listeria infection, but it has a profound impact on her fetus," says Golos, whose work is funded by the National Institutes of Health. "That's familiar now, because we've been talking about the same difference in Zika virus."
Sophia Kathariou, a North Carolina State University professor of food science and microbiology, provided a strain of listeria found to have caused miscarriage, stillbirth and premature delivery in at least 11 pregnant women in 2000. Four pregnant rhesus macaques at the Wisconsin National Primate Research Center were fed doses of the listeria comparable to what one might encounter in contaminated food. Bryce Wolfe, a UW-Madison graduate student studying cellular and molecular pathology who is lead author of the study, monitored the speed and progression of listeria's spread.
None of the monkeys showed obvious signs of infection before their pregnancies came to abrupt ends. But in tissue samples taken after each monkey experienced intrauterine fetal death, Wolfe found listeria had invaded the placenta — the connection between the mother-to-be and the fetus, which usually prevents transmission of bacteria — as well as the endometrium, the lining of the uterus.
"In that region, there's a rich population of specialized immune cells, and it is exquisitely regulated," says Wolfe. "When you introduce a pathogen into the midst of this, it's not very surprising that it's going to cause some sort of adverse outcome disrupting this balance."
"It should be a barrier," Golos says. "But we hypothesize that the maternal immune system's attempt to clear the bacteria actually results in collateral damage to the placenta, which allows the bacteria to invade the fetus."
The results suggest listeria (and perhaps other pathogens) may be the culprit in some miscarriages that usually go without diagnosed cause, but the bacteria's stealth and speed may still make it hard to control.
Golos and Wolfe plan to continue work with listeria to better define how the bacterium targets the reproductive tract, its incubation time and the problems it causes leading up to miscarriage. Their goal is to provide basic knowledge about the progression of infection and the maternal immune response to intracellular pathogens in pregnancy, which may help other researchers battling similar dangers such as Zika virus.
An electron microscope image of the bacteria: Listeria Monocytogenes.
Image Credit: CDC/James Archer