"We reasoned that this meant that a few bacteria had migrated early on from the liver or spleen to the placenta, so now both strains were in the placenta and their populations were burgeoning," Bakardjiev said. "But there must have been a much larger number that had moved from the placenta back to the liver and spleen. These would have originally been just the single strain, so their numbers skewed the ratio."

Bakardjiev and Portnoy, who is the study's principal investigator, called on Julie Theriot, associate professor of biochemistry and of microbiology and immunology at Stanford University School of Medicine, to do the mathematical modeling for the bacterial migrations. Theriot determined that only about one bacterium migrated to the placenta every five hours, while it would have taken a migration of 100,000 bacteria from the placenta to the liver to skew the numbers to the degree they found. Thus, the vast majority of the bacteria in the placenta were a result of bacterial growth there and not from migrations from the liver and spleen.

"It was surprising to find that a single bacterium is sufficient to cause placental infection," Portnoy said, "but even more surprising to find that they (the bacteria) migrated from the placenta back to the mother's liver and spleen in such large numbers."

When Portnoy and Bakardjiev ran the same experiment in non-pregnant guinea pigs, they found that 72 hours after injection, the non-pregnant animals had 1,000-fold lower numbers of Listeria in their livers and spleens than the pregnant animals, and no bacteria in their bloodstreams. In contrast, pregnant animals at 72 hours had the bacteria in their livers, spleens and blood, while the bacterial numbers continued to increase in their placentas, and their fetuses had also become infected.

"I feel that these numbers are an indication that miscarriage is a de
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Contact: Liese Greensfelder
510-643-7741
University of California - Berkeley
3-Jul-2006