Assessing the ovarian microbiome: lack of a distinguishable microbial signature beyond contamination.

Research Question: Do the ovaries harbour commensal microbes?

Design: Twenty-two ovarian tissue samples (cortex and medulla), six follicular fluid samples from gender-reassignment patients (GRP, n = 11) and five ovarian cortex tissue samples from Caesarean sections (n = 5), together with positive and negative controls, were analysed by bacterial 16S rRNA gene sequencing. Negative controls were obtained from saline storage buffer, air contaminants from the laboratory and operating room, and samples from clinicians and researchers handling the samples.

Results: Of the 33 ovarian tissue and follicular fluid samples analysed, 63.6% did not result in any bacterial DNA amplification on PCR. As a result, only 1 out of 6 follicular fluid samples from GRP resulted in DNA amplification, while two ovarian cortex and four medulla samples from GRP, together with five ovarian samples from women undergoing Caesarean section, presented detectable bacterial DNA. Following sequencing, quality filtering and decontamination, negligible microbial reads were detected in these 12 samples. Despite strict protocols to avoid contamination during sample collection and processing, a high proportion of contaminant reads (30.3-100%, median 62.1%) were found in all the samples.

Conclusions: These findings do not support the existence of a specific ovarian microbiome in the ovaries of healthy participants. It can be concluded that the incidental detection of low-biomass bacterial genera is probably attributable to contamination.