Endometrial Microbiome Profiles in Women Evaluated for Infertility or Recurrent Miscarriage: A Single-Center Descriptive Study

Background/Objectives: The role of the endometrial microbiome in reproductive failure remains incompletely understood. This study aimed to describe the composition of the endometrial microbiome in women evaluated for infertility or recurrent miscarriage. Methods: In this single-center descriptive study, endometrial samples were collected from women evaluated for infertility or recurrent miscarriage. Microbiome profiling was performed using 16S rRNA gene next-generation sequencing. Samples were classified as Lactobacillus-dominant when Lactobacillus spp. accounted for ≥90% of the total bacterial community. Alpha diversity was assessed using the Shannon and Simpson indices, while beta diversity was evaluated using Bray–Curtis dissimilarity, principal coordinates analysis (PCoA), PERMANOVA, and PERMDISP. Results: Of the 60 samples, 20 (33.3%) were Lactobacillus-dominant and 40 (66.7%) were non-Lactobacillus-dominant. Across all samples, Firmicutes was the predominant phylum (76.6%). Non-Lactobacillus-dominant samples showed significantly higher alpha diversity than Lactobacillus-dominant samples for both the Shannon and Simpson indices (p = 1.19 × 10−6 and p = 1.51 × 10−6, respectively), as well as higher observed taxa richness (p = 0.000017). PCoA based on Bray–Curtis dissimilarity demonstrated clear separation between microbiome profiles, supported by PERMANOVA (pseudo-F = 13.87, R2 = 0.193, p = 0.001). PERMDISP showed significantly greater dispersion among non-Lactobacillus-dominant samples (F = 566.94, p < 0.001). Non-Lactobacillus-dominant samples showed greater representation of Enterococcus and Prevotella. Conclusions: In this cohort non-Lactobacillus-dominant communities were more frequent with greater diversity, richness, and compositional heterogeneity than Lactobacillus-dominant communities. These findings highlight the need for larger, standardized studies with appropriate control populations to clarify their clinical significance.