No detection of relevant virus-specific DNA or RNA sequences in the placenta

The existence of a placental bacterial microbiome remains a subject of active debate, with recent studies challenging earlier claims of a resident microbial community. While the role of bacterial and viral pathogens in placental infection and adverse pregnancy outcomes is well established, the potential existence of a resident placental (non-pathogenic) virome remains largely unexplored. Given the placenta's vital role in fetal development, our study aimed to investigate whether viral genetic material is present in placental tissue, rather than to identify viral pathogens, in both uncomplicated and complicated pregnancies using viral metagenomic capture sequencing.

Placental biopsies were obtained from three pregnancy groups: (1) delivered by elective caesarean section (n = 6), (2) delivered by emergency caesarean section (n = 6), and (3) complicated by preeclampsia (n = 5). Samples were processed using VirCapSeq VERT, a targeted enrichment strategy for vertebrate viruses, followed by Illumina NovaSeq 6000 sequencing.

High quality sequencing yielded an average of 46.6 million reads per sample, with >99.6% of reads aligned to the human genome, and <0.4% of non human sequences. Across all samples, only 12 viral contigs were identified, corresponding to bacteriophages, human endogenous retroviruses, and human gammaherpesvirus 4 (not confirmed by PCR), mostly with low read counts.

Our study found no evidence supporting the presence of a resident placental virome. Together with existing data on the absence of a bacterial microbiome, these findings support the concept that the placenta does not harbor a detectable microbial or viral community under controlled sampling conditions.