An international team of researchers from Germany and Canada has investigated whether bacteria could be detected in prenatal stool (meconium) samples.
Kennedy et al. found that fetal gut colonization of healthy term infants does not occur before birth. Image credit: Darryl Leja, National Human Genome Research Institute, National Institutes of Health.
Microbial colonization of the human intestine is a key developmental process since the order and timing of microbial exposure shape the development of the gut microbiome and impact host metabolism and immunity later in life.
In humans, maturation of both intestinal barrier function and immunity occurs prenatally.
The fetal intestine is more permeable to macromolecules and less tolerant of antigens than that of term infants.
Transfer of maternal immunoglobulin G across the placenta and uptake in the fetal intestine increase near term gestation, shaping neonatal gut immune responses after birth.
To understand the mechanisms by which microbial colonization influences health later in life, scientists must know when colonization occurs.
“The key takeaway from our study is we are not colonized before birth,” said first author Katherine Kennedy, a Ph.D. student in the Department of Biochemistry and Biomedical Sciences and the Farncombe Family Digestive Health Research Institute at McMaster University.
“Rather, our relationship with our gut bacteria emerges after birth and during infancy.”
“Recent studies have sparked controversy by claiming that we are colonized by gut bacteria before birth. But studies such as these have been criticized for the ways they control for contamination.”
“By including only breech caesarean deliveries in healthy pregnant women we were able to avoid the transmission of bacteria that occurs naturally during a vaginal birth,” added senior co-author Dr. Thorsten Braun, deputy director of the Department of Experimental Obstetrics at Charité-Universitätsmedizin Berlin.
In the study, Kennedy, Dr. Braun and their colleagues examined meconium samples collected from 20 babies during breech Cesarean delivery.
Unlike first-pass meconium, no microbial signal distinct from negative controls was detected.
Additionally, positive aerobic and anaerobic clinical cultures of fetal meconium were identified as likely skin contaminants, most frequently Staphylococcus epidermidis, and not detected by sequencing in most samples.
“Recent data suggest that a person’s relationship with their own gut bacteria is most important in early life, during critical stages of immunological and physiological development,” Kennedy said.
“The fact that colonization of infants’ guts occurs during and after their births, means that not only is it vulnerable to early environmental influences, but could also offers a window of potential intervention,” added senior co-author Professor Deborah Sloboda, a researcher at McMaster University and the Canada Research Chair in perinatal programming.
“While many of the exact mechanisms surrounding gut bacteria and their role in our early development is unclear, discovering when and how we are colonized is a key first step.”
The study was published in the journal Nature Microbiology.
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K.M. Kennedy et al. Fetal meconium does not have a detectable microbiota before birth. Nat Microbiol, published online May 10, 2021; doi: 10.1038/s41564-021-00904-0
