The endometrial microbiome consists of a set of microorganisms (endometrial microbiota) that colonize the lumen of the endometrium without damaging it, at least under normal operating conditions of the immune system.

Under normal conditions, in healthy women at reproductive age, the most common colonizing bacterial species is Lactobacillus. Lactobacilli act as probiotics that can inhibit the growth of bacteria, viruses and pathogenic fungi, through a lowering of pH mediated by the production of lactic acid. In these cases the endometrium is defined Lactobacillus dominated (LD) and contains a percentage of lactobacilli > 90%.

In abnormal conditions, the percentage of lactobacilli decreases (<90%) and the percentage of others bacteria increases (> 10%). The endometrium is then defined non- Lactobacillus-dominated (NLD).

Women with idiopathic infertility are often characterized by an altered endometrial microbiota, with an increase in colonization by anaerobic bacteria such as Atopobium, Prevotella, Veillonella, Ureaplasma and Escherichia.

The most relevant example of pathology caused by an alteration of the endometrial microbiota is chronic endometritis (CE). CE is characterized by persistent inflammation of the endometrial mucosa, caused by the presence of bacterial pathogens in the uterine cavity. Because CE is often asymptomatic and not detectable by vaginal ultrasound, it is often overlooked. The prevalence of CE in infertile patients has been estimated to be approximately 39%; it has been reported as high as 60% in women with recurrent pregnancy loss (RPL) and 66% in women characterized by repeated implantation failure (RIF) ^1-2.