Microbiome in women with endometriosis and the in vitro effects of Lactobacillus reuteri on human endometrium.

UNLABELLED: Endometriosis (EMS) is a chronic inflammatory disorder affecting ~10% of reproductive-age women, with increasing evidence implicating the microbiome in its pathogenesis through immunomodulation and estrogen metabolism. This study investigated microbiome composition in the vagina, endometrium, and peritoneal fluid (PF) of women with and without EMS and further assessed the effects of Lactobacillus reuteri (L. reuteri) on endometrial (EM) cells in vitro. Samples from 41 patients were analyzed using 16S rRNA gene sequencing, targeting the V3-V4 regions. Western blotting, ELISA, and LC-MS/MS were employed to evaluate protein expression and estrogen metabolism during EM-L. reuteri co-culture with or without estradiol-17-glucuronide (E2G). Microbiome analysis revealed no significant differences in alpha or beta diversity between EMS and controls across all compartments. However, LEfSe analysis identified several taxa with differential abundance, with L. reuteri consistently altered in both vagina and EM. Across the menstrual cycle, EM and vaginal microbiomes were stable, whereas PF microbiota showed phase-dependent variation involving 60 genera and 76 species. In vitro, L. reuteri alone did not alter endometriosis-related proteins, but in the presence of E2G, it reduced BAX/Bcl-2 ratios and increased p-NF-κB, suggesting anti-apoptotic and pro-inflammatory shifts. Progesterone receptor α/β expression decreased, while estrogen receptor levels remained unchanged. L. reuteri increased β-glucuronidase activity but did not enhance E2G-to-estradiol conversion. These findings highlight L. reuteri as a potentially important species in EMS, with in vitro evidence suggesting survival-promoting effects under estrogenic conditions. Further research should explore multi-species interactions and hormonal contexts to clarify microbial contributions to EMS pathogenesis.

IMPORTANCE: Although Lactobacillus reuteri appeared more abundant in the vagina and endometrium of controls, suggesting a protective role, in vitro findings paradoxically indicated anti-apoptotic and pro-inflammatory effects under estrogenic conditions, underscoring the need for further investigation of multi-species microbial interactions and hormonal contexts in endometriosis pathogenesis.