P-715 NGS-based metagenome analysis of endometrial microbiome in women with implantation failure after in vitro fertilization: results of a prospective cohort study

Abstract                                   Study question                   We aimed to compare the endometrial microbiome between pregnant and non-pregnant women after frozen embryo transfer (FET) with euploid embryos.                                                   Summary answer                   Specific bacteria taxa had significantly higher relative abundance in the endometrium of patients with implantation failure after FET with euploid embryos.                                                   What is known already                   Several studies revealed a significant association between the endometrial microbiome composition, including the abundance of Lactobacilli and pathogen species and the occurrence of implantation failure or miscarriage. However, there is still scarce information about the influence of relative abundance of certain bacteria taxa on the pregnancy outcome in patients with idiopathic infertility and FET.                                                   Study design, size, duration                   In this prospective observational cohort study endometrial biopsies were collected from 30 women aged between 34 and 45 during the mid-luteal phase (LH + 7) in a natural cycle. FET was performed with euploid embryos (after preimplantation genetic testing for aneuploidy (PGT-A)), up to 6 months after the biopsy. The exclusion criteria for the patients were history of recent inflammatory disease, chronic endometritis, recent antibiotic treatment, moderate or severe endometriosis, adenomyosis, uterine hyperplasia, and endometrial polyps.                                                   Participants/materials, setting, methods                   Endometrial microbiota composition was analyzed using 16S rRNA (v4-v5 region) next generation sequencing (NGS). The sequencing data was assigned to bacterial taxonomy and the background-contaminated bacteria were excluded from the analysis. The Student t-test or Mann-Whitney U test for the relative abundance of each bacteria taxa were applied according to normal or not normal distribution of data. A Bonferroni correction was applied to the significant levels obtained.                                                   Main results and the role of chance                   The performed analysis of 30 patients with different clinical outcomes, including no pregnancy (n = 16), clinical pregnancy confirmed by ultrasound (n = 14) revealed differences in the endometrial microbiome composition.                   Baseline characteristics including age, BMI and duration of infertility were comparable between the studied groups of patients.                   In total, 271 distinct bacterial species and 668 bacterial genera were identified. The number of unique species, found in non-pregnant women was 62 (22.88%), while in pregnant patients it was 39 (14.39%). Among them, bacteria with high frequency of occurrence such as Cutibacterium granulosum, Isoptericola, Acetomicrobium, Marivivens, Syntrophomonas and Bacteroides were found only in patients with unsuccessful implantation while Bosea was present only in women with successful implantation.                   Based on the analysis of bacteria relative abundance, Lactobacillus was the most prevalent genus in both groups. The abundance of this genus was not significantly different between the studied groups. In contrast, Serratia marcescens, Delftia, Massilia, Glutamicibacter, Lactococcus and Staphyloccocus were significantly enriched in the non-pregnant group (p &amp;lt; 0.05).                                                   Limitations, reasons for caution                   Our study is a single-center study with a relatively small sample size.                                                   Wider implications of the findings                   The microbiome showed specific composition in pregnant compared to non-pregnant women. It could be hypothesized that an appropriate treatment for optimization of endometrial microbiome content in women with diagnosed microbiome dysbiosis could be beneficial for improvement of pregnancy rates.                                                   Trial registration number                   The current research was funded by National Science Fund, Ministry of Education, Bulgaria, Contract № KP-06-N53/14/16.11.2021.