Endometrial Microbiome and Its Correlation to Female Infertility: A Systematic Review and Meta-Analysis

The endometrial cavity was considered sterile until the second half of the 20th century. Through modern technological advances and the sequencing of the bacterial 16S rRNA gene, it was proven that the area possesses its own unique microbiome, which can be categorised into two types, Lactobacillus-dominant (LD, with a Lactobacillus spp. abundance percentage greater than 90%) and non-Lactobacillus-dominant (non-LD, with a Lactobacillus spp. abundance percentage smaller than 90%), with other species like Bifidobacterium, Gardnerella, Prevotella, and Streptococcus also being prominent. The aim of this study was to investigate the possible correlation of the endometrial microbiome to female infertility, through the identification and appraisal of studies published in the databases PubMed, Web of Science, and Scopus. Moreover, 12 studies met the research criteria, including the analysis of endometrial fluid or tissue samples from infertile women through PCR, culturomics-based, or NGS methods. According to most of these studies, a eubiotic LD-type microbiome seems to be best for maximising endometrial receptivity and pregnancy chances, whereas a dysbiotic non-LD-type microbiome, with increased α-diversity and a higher number of pathogens, has a harmful effect. There were few studies that presented contradictory results without, however, a satisfactory explanation. Thus, more time and a greater number of studies are required to clarify contradictions and achieve more certain results.