Results of a UK study show that while intrauterine infection may contribute to preterm birth (PTB), a reproducible “preterm placental microbiome” does not exist. The findings, published in Applied and Environmental Microbiology, are from DNA analysis of placental samples from both PTB and term deliveries.
For the case-control study, the researchers used 16S targeted amplicon sequencing to evaluate differences in placental microbiota in term and PTB deliveries in a large UK pregnancy cohort. DNA was extracted from 400 placental samples from 256 singleton pregnancies and analyzed for bacterial DNA. The samples were from spontaneous preterm births (sPTB), PTB, and term deliveries and most (89%) were from parenchyma, but for a few pregnancies, matching villous tissue also was available.
The authors hypothesized that a distinct microbial profile would be seen in samples from sPTB versus those from term deliveries. What they found was widespread contamination across all placental samples, which implicated both delivery method and reagent contamination. In sequenced reads, 136 contaminating operational taxonomic units (OTUs) from 44 genera were found. Thirty-two of the genera (73%) had previously been reported as reagent contaminants.
After contaminant reads and low-abundance samples were removed from the dataset, the researchers found that Lactobacillus crispatus was the most abundant OTU in the remaining samples, found in 59.4% of them. Eight of the top 20 OTUs in the filtered dataset mapped to the Lactobacillus genus. In placental tissue from vaginal deliveries, Lactobacillus and Bacteroides were the most common genera seen, whereas Streptococcus and Corynebacterium were most abundant in placental tissue from cesarean deliveries.
Six genera were significantly more abundant (P < 0.01) in placental tissue from sPTB versus PTB (Ureaplasma, Prevotella, Salnicoccus, Mycoplasma, Capnocytophaga, Anaerococcus) and 7 genera were more abundant in sPTB versus term samples (Tepidomonas, Salnicoccus, Capnocytophaga, Mycoplasma, Anaerococcus, Truepera, Coprobacillus). After adjustment for delivery method, recruiting hospital, maternal ethnicity, body mass index, smoking and tissue type, 4 genera were significantly more abundant in the tissue from sPTB versus PTB (P < 0.01) (Mycoplasma, Ureaplasma, Mogibacterium, Salnicoccus). A similar difference was found for abundance of 8 genera in sPTB versus term delivery (Anaerococcus, Capnocytophaga, Coprabacillus, Erwinia, Mycoplasma, Salnicoccus, Turicibacter, Tepidimonas).
The authors acknowledged that their study was limited, in that the placental samples were not originally collected for use in microbial analysis. They said theirs is one of the largest cohorts of 16S sequenced placental tissue from SPTB in the literature. Their findings, they believe, “suggest that there may be a low-level non-pathogenic placental microbiome present in many, if not all, placentas. However, differentiating this from organisms picked up at delivery, or during experimental handling is an on-going challenge.”