Time course of fetal tissue invasion by Listeria monocytogenes following an oral inoculation in pregnant guinea pigs

Pregnant women are 20 times more likely to develop listeriosis than are members of the general population, and infection can result in abortion, stillbirth, or neonatal illness. The objective of this study was to orally challenge pregnant guinea pigs with Listeria monocytogenes to assess maternal and fetal tissue invasion at postinoculation days 2, 6, 9, and 21. The time course of invasion was followed by fluorescence microscopy and a traditional culture method. Guinea pigs were treated on gestation day 35 with L. monocytogenes doses ranging from 10(4) to 10(8) CFU. L. monocytogenes was isolated and viewed in maternal and fetal tissues as early as 2 days postinoculation. L. monocytogenes was isolated from placentas, fetal livers and brains, and maternal spleens at similar rates, suggesting that invasion of the spleen could be indicative of fetal invasion. When comparing fecal shedding, all animals treated with 10(4) CFU were shedding L. monocytogenes by postinoculation day 7, and all animals treated with the higher doses (10(6) or 10(8) CFU) were shedding L. monocytogenes by postinoculation day 5. These data suggest that L. monocytogenes crosses the fetoplacental barrier and invades the fetus by day 2 after maternal ingestion. When comparing the sensitivities of microscopy and culture, neither method consistently detected L. monocytogenes at a higher rate. However, detection in individual tissues differed. Microscopy was significantly more sensitive with fetal liver (P<0.001) and brain (P<0.001) at the highest dose of 10(8) CFU, but at the lowest dose of 10(4) CFU culture was significantly more sensitive with maternal spleen (P=0.04).