Proof of Principle: Transfer of Small Numbers of Bacteria from Mother to Fetus

Abstract

The fetus is thought to develop in a sterile environment in utero. Long standing dogma that “the uterus and the feto‐placental unit is “sterile” is based primarily on microbiological culture‐based techniques that were unsuccessful in growing “culture resistant” bacteria or intracellular bacteria. We have reported the presence of low numbers of bacteria in tissues from normal sheep fetuses in pregnancies not complicated by infection. The exposure of the fetus to bacteria might aid neonatal survival by informing fetal immune development. We propose that the fetus is not sterile and that bacteria or fragments of bacteria can be transferred from mother to fetus. We hypothesize that inoculation in the maternal mouth results in the appearance of bacteria in the fetus. We used S. aureus containing green (GFP), red (RFP), or far‐red (FRFP) fluorescent protein‐expressing plasmids to inoculate late‐gestation pregnant sheep (gestational age= 130–135 days, n=7) intravenously (GFP, 104 cfu), into maternal mouth (RFP, 104 cfu) and vagina (FRFP, 104 cfu). These were small doses of bacteria which did not cause physiological (no fever) or psychological (no anorexia) signs of infection. Five to seven days post maternal inoculation, animals were humanely sacrificed, and fetal tissues were collected, and DNA was extracted from placenta and fetal liver, spleen, and cerebral cortex. We probed for GFP plasmid using several primer pairs for endpoint PCR. While detection of whole‐length plasmids was not successful, PCR reactions probing for smaller fragments of plasmid were successful. We found GFP plasmid DNA in all tissues in 10/10 fetal livers and RFP in 10/10 livers. The appearance of GFP and RFP‐labelled bacteria in fetal liver (p=7.497e‐06) was statistically significant as tested by Chi‐Square analysis. We did not detect significant FRFP plasmid DNA in liver. Analysis of tissues by immunohistochemistry revealed GFP and RFP‐expressing bacteria in fetal tissues, although most appeared to be in aggregates. We conclude that S. aureus, introduced in small numbers in maternal mouth and bloodstream, appear in the fetus and placenta. We were unable to determine whether these bacteria were alive in the fetus.Support or Funding InformationThis work was supported by HD033053, AI120195, and HL083810This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.