Pregnancy reduces the genetic resistance of C57BL/6 mice to Listeria monocytogenes infection by intragastric inoculation

In this study, we compared genetically resistant C57BL/6 and susceptible A/J mice for their resistance to Listeria monocytogenes infection during pregnancy. Intragastric infection with modest numbers of bacterial cells (105 CFU) caused reproducible fetal infection and abortion in both mouse strains. Bioluminescence imaging demonstrated dissemination of L. monocytogenes cells from maternal to fetal organs within 3 days of intragastric infection. Although non-pregnant C57BL/6 mice were significantly more resistant to infection than non-pregnant A/J mice, C57BL/6 and A/J mice had similar microbial loads (CFU) in maternal and fetal tissues during pregnancy. Inflammation and necrosis, however, were more severe in A/J mice as evaluated by semi-quantitative histopathology. Although the microbial load in fetal tissues was similar for all fetuses within a single uterus, inflammation and necrosis varied among individual fetuses and placentas. We also noted that the uterus is a target for L. monocytogenes infection in non-pregnant mice.