Marilen Benner

CHAPTER 6 148 Franasiak and colleagues characterized the endometrial microbiome of 33 women by collecting material from the tip of the catheter used for embryo transfer (ET) during ART (57). Both in the group of non-ongoing and ongoing pregnancies, Lactobacillus and Flavobacterium were identified as the most abundant genera. However, in this initial study only women undergoing ART for unspecified reasons were included. Therefore, it is unclear to what extent these findings represent the colonization of a healthy, fertile endometrium. The same holds true for the study by Tao and colleagues (22) assessing the microbiota obtained from catheter tips during ET of 70 women. Lactobacillus was abundantly detected (>90% of OTUs in 33 women, >70% in 50 women). Samples also frequently contained Corynebacterium (40 women), Staphylococcus (38 women), Streptococcus (38 women) and Bifidobacterium (15 women). To achieve reliable sequencing results, particular attention was given to sample preparation, as sampling using remaining tissue from catheter tips only yields ultra-low bacterial cell counts. The authors were able to reliably determine bacterial abundance when more than 60 bacterial cells were present per sample, although no estimation was given of how many cells the catheter tips likely contain. Verstraelen and colleagues identified Bacteroides as the dominant genus, present in >90% of the women that were included. All women underwent hysteroscopy in the absence of uterine anomalies (16). Again, the cohort of women employed sought medical help due to recurrent implantation failure (RIF) or recurrent pregnancy loss. Twelve of the 19 women included showed 75% similarity in bacterial composition (Bray-Curtis dissimilarity of 24.6%, range 13.2 – 34.3%) due to general high abundance of Bacteroidetes and Proteobacteria taxa. Next to this core of Bacteroidetes and Proteobacteria, five women also presented with L. iners (n=2) , P. amnii (n=1) or L. crispatus (n=1) as the most abundant species. Large dissimilarities were observed in two women (79.0 and 90.7% Bray-Curtis dissimilarity) showing Prevotella spp. or L. crispatus dominance, which the authors ascribe to possible contamination from the vagina. Control swabs from the vagina could have given an indication if the distinctive microbial profile of these women indeed reflected an artifact from the transcervical sampling approach but no such data are available. In a study focusing on the relation of endometrial polyps to local microbiota, Fang et al. included 10 fertile women as a control population in their 16S analysis of transcervical uterine swabs. Proteobacteria, Firmicutes, and Actinobacteria were consistently found at phylum level; Enterobacter, Pseudomonas, and Lactobacillus at genus level. Moreno and colleagues (2016) were also able to include fertile women (n=13 for comparison of vaginal aspirates and EF, n=22 for compositional differences between pre-receptive and receptive phase) of reproductive age (19). Lactobacillus (71.1%), Gardnerella (12.6%), Bifidobacterium (3.7%), Streptococcus (3.2%), and Prevotella (0.9%) were the most identified genera. Endometrial microbial composition was categorized into either Lactobacillus -dominated (>90% Lactobacillus spp . ) versus non- Lactobacillus dominated (>10%bacteria other than Lactobacillus, such as A. vaginae, G. vaginalis,