Exploratory evaluation of the bacterial load of uterine microbiota: Potential implications for implantation and treatment response in assisted reproductive technology.

The clinical impact of endometrial microbiota on assisted reproductive technology (ART) primarily relates to the supremacy of Lactobacillus and its species composition. However, bacterial load, reflecting total microbial biomass, may be another key parameter influencing ART outcomes. This study aimed to evaluate whether bacterial load is a useful metric for assessing the endometrial environment and predicting ART success. This retrospective study included 223 women with recurrent implantation failure (RIF). Endometrial samples were collected and analyzed using 16S rRNA gene sequencing to determine microbiota composition and Lactobacillus abundance. The bacterial load was quantified via qPCR. Patients were categorized as Lactobacillus-dominant (LD) or non-Lactobacillus-dominant (NLD). We assessed the associations between bacterial load, implantation, pregnancy outcomes, and therapeutic response in NLD. Additionally, the optimal bacterial load range and related host factors were evaluated. A higher bacterial load was associated with LD status and a better treatment response in NLD. Although implantation and ongoing pregnancy rates were not significantly correlated with bacterial load, a potential link with pregnancy outcome was observed. Body mass index (BMI) and age were not associated with bacterial load variability. In conclusion, a quantitative assessment of bacterial load may complement microbiota composition analysis in evaluating endometrial environments for ART.