Distinct Virome and Bacteriome Profiles of Term and Preterm Placentas from African, Asian and European Women

While the human placental microbiome has been extensively studied, information regarding the virome remains limited. The association between subclinical placental viral infections and spontaneous preterm birth (PTB) is poorly understood. Hence, we examined fresh placenta samples from women in the UK, Bangladesh, and South Africa to elucidate the virome profiles and association with spontaneous PTB. We employed shotgun metagenomics, real-time PCR, and Gram staining, complemented by bioinformatics, to detect both viruses and bacteria in the decidua and chorionic villous tissues. We did not identify any known pathogenic viruses in 236 placental samples, except for one preterm placenta from the Bangladeshi cohort, where human herpesvirus 6 was detected. The majority of the viruses were bacteriophages present at very low abundance and low frequencies across the three cohorts. Lambdavirus lambda was detected in 14% of UK and 11% of Bangladeshi samples but was absent in South African samples. Papiine betaherpesvirus 3 was identified in 62.5% of South African placentas. Punavirus (29.1%) and Streptococcus phages (16.67%) were also prevalent in South African samples. A variety of bacterial species including Moraxella osloensis, Delftia lacustris, Cutibacterium acnes, Escherichia coli and Staphylococcus epidermidis were detected in samples across the three cohorts in variable frequencies but without any notable tissue immune responses. None of the viruses or bacteria exhibited associations with PTB, except D. lacustris, which was significantly more abundant in preterm compared to term placentas in the Bangladeshi cohort. We conclude that the human placenta harbours a virome predominated by bacteriophages and that subclinical viral infection in spontaneous PTB is rare.