Marilen Benner

MICROBIOTA AND ENDOMETRIAL HEALTH 165 6 Controlling for vaginal and environmental contamination A transvaginal approach carries the risk of contamination by the cervicovaginal microbiome. Due to the high biomass of the vaginal relative to the uterine microbiome and the high abundance of Lactobacillus, misrepresentation of species distribution cannot be excluded. To achieve more conclusive results in the near future, transvaginal studies on endometrial microbiota need to strictly incorporate control samples from vagina and cervix. Additionally, contamination by reagents needs to be controlled for by including controls of all extraction kits used, a common source for misleading data (195). Also, air swabs and swabs of other environmental sources of microbial exposure need to be taken into account when trying to establish conclusive results on the low-biomass microbiome of the uterus. Standardized sample processing In the presented studies, major differences in the processing of material, data collection and analyses are all sources of variation (196). The choice of hypervariable region (V1-V9) of the 16S rRNA, containing species-specific sequences used for assessing microbial diversity, influences the detection results. V1 for example was found to be well suited to distinguish between S. aureus species, whereas V2 was more successful in Mycobacterium species differentiation (56). Depending on the variable region of the 16S rRNA, different species are preferentially detected. A standardized approach in which study design, sampling method, DNA extraction with generation of 16S amplicons and sequencing, assigning of OTUs to genus/species, and reporting of the findings is needed (197) to achieve conclusive results for endometrial and placental microbiome studies. Without these standardized, well-controlled study designs the uterine microbiome will still not come into its own in the future. Points of interest Timing of intervention Owing to the dynamic fluctuations of microbiota in the uterus, the timing of microbial influence(s) on a healthy pregnancy needs attention. Depending on when the pregnant (or soon-to-be pregnant) woman undergoes any external microbial (pro-/pre or antibiotic) intervention, this might alter the physiologic processes of implantation, placenta formation or placenta maintenance if microbiota are involved. There is increasing evidence that BV is associated with infertility and pregnancy complications (198). Interestingly, the risk of delivering preterm because of BV is not reduced after clearance of infection by antibiotics (199). This might be explained by the fact that the effects of BV have already taken place and the initiated cascade of changes induced by the bacterial imbalance cannot be reversed. Accordingly, treatment of BV before 20 weeks of gestation might decrease the risk of BV-associated preterm birth, suggesting that the early phase of pregnancy in particular is sensitive to microbial impact (200). Systematic reviews on other conditions of bacterial imbalance associated with pregnancy and their treatment might