In utero transmission of Mycoplasma pulmonis in experimentally infected Sprague-Dawley rats

Abstract

Genital mycoplasmosis is important as an animal model for the interaction between infectious agents and the host during pregnancy as well as in its own right as a confounding variable affecting research projects in which the rat is used as a model to study reproductive function and physiology. We report the in utero transmission of Mycoplasma pulmonis and the development of placentitis, amnionitis, and mild fetal bronchopneumonia in Sprague-Dawley rats. A minimum of 10 days prior to breeding, specific-pathogen-free female Sprague-Dawley rats were infected by intravaginal inoculation with 3 x 107 CFU ofM. pulmonis X1048 or with an equal volume of sterile broth. Rats and fetuses were subjected to necropsy at days 11, 14, and 18 of gestation. M. pulmonis was able to invade the placenta, cross the placental barrier, and establish an amniotic fluid infection by gestational day 14. It was isolated from the oropharynx and lungs of fetuses at gestational day 18. The placenta was more frequently colonized than amniotic fluid, followed by the fetal oropharynx and lungs, supporting an ascending route of infection. Histopathological evidence also support an active infection, with lesions compatible with placentitis, amnionitis, and mild fetal bronchopneumonia. M. pulmonis can traverse the placenta, resulting in infection of the amniotic fluid and in utero transmission of the microorganism to the developing fetus.

Hematogenous infection of Sprague-Dawley rats with Mycoplasma pulmonis: development of a model for maternal and fetal infection

“…For each experiment, the number of CFUs in each inoculum was verified by culture. Unless otherwise noted, all cultures taken from animals at time of necropsy were serially diluted 10-fold in Frey's broth to 10 Ϫ5 and processed as described previously [22][23][24][25][26] to determine number of CFUs. Selected cultures were confirmed as M pulmonis by polymerase chain reaction.…”

“…Necropsy procedures were performed as previously described. 22,25,26 Rats were killed with an overdose of sodium pentobarbital (180 mg; Veterinary Laboratories Inc, Lenexa, TX) injected intraperitoneally. The spleen, liver, trachea, lung, vagina, and uterus were aseptically removed from each dam and cultured for M pulmonis.…”

“…AF from each fetal unit was obtained by puncturing the chorioallantoic membrane with a sterile needle. 22,25,26 The fetus was disinfected with 70% ethanol before aseptic dissection and the fetal brain, lung, and spleen/liver was removed and minced separately for culture.…”

“…Colonization of SD rat dam and fetal tissues by M pulmonis occurred in a dose-dependent manner after IV inoculation, similar to colonization results seen previously in vaginally and IV-infected rats. 20,22,25,26 The minimum threshold dose required to establish infection of the dam and fetus is at least 10 3 CFU, with consistent colonization of all tissues achieved only with 10 7 CFU. For rat fetal units, the 10 5 dose may be too low to achieve ID 50 , but this dose would allow comparisons to be made between infected and uninfected fetal tissues within the same dam.…”

“…21 Further, in rats experimentally infected in the vagina before breeding, the reproductive tissues were more commonly colonized than the trachea once pregnancy was established. 22,25,26 The current paradigm is that infectious agents colonize the placenta, breach the placental barrier, and infect the AF, thereby gaining access to the fetus directly or via hematogenous spread from the placenta. 28, 30 Our results strongly support the concept that the SD rat fetuses may have been directly infected via the labyrinthian circulation.…”