Kinetics of placental colonization of mice inoculated intravenously with Brucella abortus at day 15 of pregnancy.

When 15-day pregnant mice were challenged i.v. with 1 to 2 X 10(4) virulent Brucella abortus Strain 544, the bacteria were recovered from all the placental discs and spleens of the dams, as few as 5 min post-challenge. Kinetics in both organs differed greatly. In spleens, the fraction of the inoculum recovered increased from 3-20%, from 5 min-6 h post-challenge. Brucella multiplication was low from 24-72 h. In each placenta, there was first an immediate dissemination of less than 0.1% of the inoculum. Then, some brucella disappeared leaving about 20% of the placentas free of infection, the others being infected with as few as 3 Brucella CFU on average, 4-6 h post-challenge. Brucella then multiplied in colonized placentas, leading to an average of 10(4) brucella CFU per placenta at 72 h post-challenge.