Presence and Dominance of Lactobacillus in the Endometrial Microbiome and Age-Related Associations in Patients with Recurrent Reproductive Failure

Objectives: To evaluate the presence and dominance of Lactobacillus in the endometrial microbiome and their age-related associations in a large group of Bulgarian patients with recurrent implantation failure (RIF) and recurrent pregnancy loss (RPL) who attend our clinic. Methods: This retrospective study included 199 patients (mean age: 35.69 ± 5.16) with RIF (n = 103) and RPL (n = 96) who visited our fertility clinic between October 2019 and November 2022. Endometrial samples were analyzed using real-time PCR for target DNA sequences. Results: Overall, 62.8% (n = 125) exhibited an absence of Lactobacilli in their endometrial samples, with 63.1% (n = 65) of the RIF group and 62.5% (n = 60) of the RPL group showing a lack of Lactobacilli, with no statistically significant difference between the groups (p = 0.926). A Lactobacillus-dominant microbiome was found in 23.6% of the entire cohort (n = 47), 25.2% of the RIF group (n = 26) and 21.9% of the RPL group (n = 21). A reduced abundance of Lactobacilli was identified in 13.5% of the cohort (n = 27), though to differing degrees. There was no significant relationship between the abundance of Lactobacilli and belonging to the RIF or RPL group. A statistically significant difference was found in the mean age of two groups in cases with a Lactobacillus-dominant microbiome (mean age of 36.4 ± 4.8 years in the RIF group and 32.5 ± 3.5 years in the RPL group) (p = 0.004). Conclusions: Our findings demonstrate a high prevalence of non-Lactobacillus-dominant microbiomes in a large group of Bulgarian patients with RIF and RPL and significant age-related Lactobacillus changes in the microbiome of patients with RPL. These results point to the potential role of the uterine microbiome and support the need for further prospective studies, especially in cases of advanced maternal age.