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Table 1 Summary of recent studies on the microbiology of amniotic fluid, placenta, and meconium

From: A critical assessment of the “sterile womb” and “in utero colonization” hypotheses: implications for research on the pioneer infant microbiome

Sample type Population Methods Significant findings and/or authors’ conclusions Reference
Placental membranes, umbilical venous blood Term and preterm vaginal and elective cesarean deliveries, (preterm deliveries include pregnancies complicated with preeclampsia, fetal growth restriction, or prolonged labor), deliveries that presented PPROM (n = 52) FISH using generic probes for 16S rRNA genes Bacteria were detected in 70% of placentas. Authors concluded presence of bacteria is common in placental membranes, but insufficient to cause preterm labor or PPROM [37]
Meconium passed within the first 2 h of life Term healthy newborns (n = 21) Culture methods, Gram staining, 16S rDNA sequencing Bacterial species isolated from one single meconium samples varied from 1 to 5. Enterococcus faecalis was the most abundant species found in 80% of the samples. Samples clustered by processing time [8]
Placental membranes Full-term and preterm vaginal and cesarean deliveries; preterm deliveries with and without PROM (n = 74) Standard PCR of 16S rRNA gene and quantitative PCR for selected bacteria Bacterial DNA was detected in 30% of placental tissue by standard PCR, while 43% were positive by qPCR; 14% were positive by both methods. No bacterial DNA was detected in C-section deliveries at term, while 50% of term vaginal deliveries were positive [93]
Placenta Full-term vaginal and cesarean deliveries from pregnant women participating in diet study (n = 34) Aerobic and anaerobic cultures. PCR of 16 s rRNA gene using genus and species specific primers for Lactobacillus and bifidobacteria DNA was detected in 94% of samples by PCR. Bacteria of interest were not detected by culture methods [84]
Meconium and feces Preterm neonates (n = 23) 454 pyrosequencing Bacterial DNA was detected in 91% of samples. Lower gestational age was associated with lower bacterial diversity, but there are no differences in diversity between C-section and vaginally delivered infants [98]
Amniotic fluid, placenta and meconium Elective cesarean deliveries of healthy mothers enrolled in probiotic study (n = 43) Quantitative PCR for selected bacterial groups (Lactobacillus, Bifidobacterium, Bacteroides, Clostridium leptum group) Lactobacillus DNA was found in 100% of placentas, Bifidobacterium in 41%, Bacteroides in 34% and Clostridium leptum in 31%. DNA for the selected bacterial groups was found in 43% of amniotic fluid samples [51]
Meconium passed between 2–48 h after birth Infants born by vaginal or cesarean deliveries from diabetic and non-diabetic mothers (n = 23) 16S rRNA sequencing using Pacbio RS system Bacteria were found in 100% of samples. Diversity is lower in meconium when compared to adults; higher in infants from diabetic when compared to non-diabetic mothers [99]
Meconium Healthy full-term deliveries (n = 20) Pyrosequencing of 16S rRNA gene Meconium microbiota differed from the microbiota of feces, vagina, and skin from adults but was similar to that of young infant feces. Meconium microbiota has an intrauterine origin and is influenced by maternal factors. [149]
Maternal feces, meconium, baby’s feces at different timepoints Healthy mothers, full-term pregnancies, all infants exclusively breastfed for at least 2 months (n = 17) Culture methods, PCR of 16 s rRNA genes, qPCR using Bifidobacterium species-specific primers Bifidobacterium species were found in all newborn/infant samples. Vaginally delivered mother-infant pairs showed monophyletic bifidobacterial strains, while none of the strains identified from C-section pairs were identified as monophyletic [135]
Placental basal plates Term and preterm deliveries with and without history of PROM, chorioamnionitis, group B Streptococcus infection, sexually transmitted infection, and/or UTIs (n = 195) Histology using H&E, Gram staining, hema 3 (modification of Giemsa stain) and Brown-Hopps modification of Gram stain 27% of placentas contained intracellular bacteria in basal plate. No difference found in the incidence of bacteria in chorioamnionitis, PTB, or group B Streptococcus infection. There was a twofold risk increase for intracellular bacteria in very preterm birth [38]
Meconium passed between birth and 48 h after birth Vaginal or cesarean deliveries of preterm neonates (n = 52) Ion torrent sequencing of 16S rRNA genes 67.3% of samples showed amplification of the 16S rRNA gene. Gestational age had a greater influence than mode of delivery on microbial community structure. Meconium is indicative of amniotic fluid bacterial communities [107]
Placenta Healthy pregnancies compared to preterm birth and history of antepartum infection (n = 320) Illumina sequencing with of 16S rRNA genes and WGS metagenomics Placentas are not sterile. Placental microbiome is associated with remote history of antenatal infection. Microbial profiles resemble oral microbiome. [9]
Placental membranes (chorion and amnion) Term vaginal deliveries, preterm spontaneous vaginal deliveries positive for chorioamnionitis and cesarean deliveries with intact membranes (n = 24) Roche 454 FLX pyrosequencing of 16S rDNA There was increased frequency of bacterial detection and wider spectrum of bacteria in preterm placental membranes than in term deliveries [50]
Placental tissue, venous blood, urine, amniotic fluid Normotensive and preeclamptic primiparous (n = 110) Standard PCR and Illumina sequencing of 16S rRNA genes 12.7% of placental tissue samples from women with preeclampsia were positive by PCR, while all normotensive women were negative. Blood, urine, and amniotic fluid samples were negative except for one amniotic fluid sample colonized by Bacillus cereus [150]
Posterior and side wall of vagina, inner surface of placenta, and meconium One vaginal delivery and one cesarean delivery (n = 2) Pyrosequencing of 16S rRNA genes Placentas are not sterile. Placental and fecal samples have more diversity than vaginal samples [151]
Meconium passed between 3 and 23 h after birth Full-term, healthy vaginally delivered infants exclusively breastfed (n = 15) FISH, standard PCR Bacteria were detected in 66% (10 of 15) of meconium samples using FISH and 7% (1 of 10) by PCR. A higher percentage of sterile samples is observed in samples with lower MIC [14]
Meconium, maternal and infant feces, colostrum, placenta, amniotic fluid Full-term mother-infant pairs submitted to elective C-section (n = 15) Cultures, 16S rRNA pyrosequencing, qPCR, DGGE There were 41 bacterial phylotypes shared between meconium, amniotic fluid, and placenta. Bacterial communities of meconium and colostrum share a common maternal source; colostrum does not directly contribute to the meconium microbiota [10]
Meconium Vaginally or cesarean-delivered healthy full-term Japanese infants (n = 151) RT-qPCR for selected species Bacteria were detected in 95% of meconiums. The infant microbiota is strongly influenced by delivery mode. However, these differences are not noticeable at meconium stage, but become prominent at a later stage [92]
  1. DGGE denaturing gradient gel electrophoresis, DNA deoxyribonucleic acid, FISH fluorescence in situ hybridization, PCR polymerase chain reaction, PROM premature rupture of membranes, PPROM preterm premature rupture of membranes, qPCR quantitative polymerase chain reaction, rDNA ribosomal deoxyribonucleic acid, rRNA ribosomal ribonucleic acid, UTIs urinary tract infections, RT-qPCR reverse-transcription quantitative polymerase chain reaction