Marilen Benner

MICROBIOTA AND ENDOMETRIAL HEALTH 163 6 of increased barrier permeability, more barrier breach of the uterine microbiota can occur. This in turn can lead to a more pro-inflammatory immune environment with mucosal immune cells stimulated to secrete the cytokines that are beneficial for the peri-implantation period. DISCUSSION Conclusions on the possible presence of uterine colonization Even before the era of 16S RNA analysis, it was put forward that “it is difficult to envision that a mucosa that is continuously exposed to microorganisms present in the lower genital tract and that is regularly invaded by sperm that can carry microorganisms into the endometrial cavity, may be free of bacteria and its products such as endotoxin” (193). We also agree that, based on the various lines of evidence for the existence of an endometrial microbiota, the era of the sterile uterus, free of any microbial compounds, has to come to an end. Taken together, this variety of immunological adaptations due to microbial presence contributes to establishing the right balance of tolerance versus reactivity towards bacterial ligands, and likely has a physiological impact on pregnancy (Figure 2). Following the suggestion by Espinoza and colleagues, it can be envisioned that as long as no pathologic pro-inflammatory response against the microorganisms of the endometrial mucosa is started, the presence of a certain microbial coexistence does not harm the conceptus (194), and might well contribute to vital processes in the peri-implantation period and pregnancy. Additional studies required to define the core microbiome Sample cohort Even though several studies have evaluated the microbiome of the endometrium, a “baseline” or “core” microbiome of this healthy endometrium has not been defined. The studies presented here on 16S rRNA evaluation of endometrial microbiota vary to a large extent in experimental approach. Differences in sample population, sample acquisition method, 16S region, controls, and bacterial genotyping make it impossible to combine the data in a more powerful meta- analysis. Of note, all studies enrolled small and highly selected cohorts. Except for the studies by Fang and colleagues (2017) assessing 10 healthy women, and Moreno and colleagues (2016) including 22 healthy women, all studies enrolled subjects seeking treatment because of reproductive complications or uterine abnormalities (18, 19). Therefore, the majority of data does not reflect a putative steady-state healthy endometrial microbiome. Only recruitment of healthy, fertile women would provide insight into the microbiome of a fertile, receptive endometrium. However, without underlying medical reasons, sampling in healthy women is unethical and therefore a major limitation to investigating uterine microbiota.