Endometrial microbiota: role in embryo implantation failure in PMA cycles

The articles in the “Expert Opinion” section cover some of the most important and debated topics in their respective clinical areas. Because of the level of depth attained, the texts may contain very complex terms and concepts. The use of the glossary may help in understanding these articles, and other, more popular site content will help clarify the topics covered.

The endometrial microbiota is the population of microorganisms that colonizes the endometrial mucosa, tissue that covers the inner wall of the uterine cavity, the most superficial part of which periodically undergoes those desquamative changes that are characteristic of the menstrual cycle. Part of the microbiota are the totality of microorganisms present: bacteria, fungi, archaeobacteria, protozoa and viruses that metaphorically represent a photographic snapshot capturing the population residing in a limited space, at an arbitrarily chosen instant. The microorganisms of the microbiota, physiologically, or sometimes pathologically, live in symbiosis with the human body [1]. Thus, the uterus represents a microbiological niche within which there is a complex environment dominated by Firmicutes, Bacteroidetes, Proteobacteria, and Actinobacteria, the particular balance of which is beneficial for maintaining a healthy microenvironment [2]. Conversely, altered microbial composition may have a negative implication for the development of gynecological diseases and in the failure of reproductive desire [3].

For many decades, the uterus was thought to be a sterile environment despite being contiguous to the vagina, an organ rich in bacterial flora; cervical mucus was thought to maintain uterine sterility. Only recently has this dogma been challenged. The main theory is that of microbial ascension from the vagina. Although a kind of cervical plug (thick obstructing mucus) is known to protect the uterine environment from invasion attempts, during intercourse some sperm are able to ascend inside the uterus through small channels formed in the context of the mucus [4]. In addition, other studies have shown evidence for the existence of a uterine pump that moves particles from the vagina to the cervical canal and finally into the uterine fundus within 15 minutes of intercourse [5]. Other possible methods of bacterial spread include their hematogenous migration and transmembrane passage from the intestine to the peritoneal cavity with retrograde ascension of bacteria through the fallopian tubes. Dendritic cells and leukocytes also transport bacteria in the intestine and can spread them hematogenously to other districts, and finally to the endometrium

The advent of next-generation sequencing (NGS) technologies has made it possible to study ribosomal RNA (rRNA)16S sequences, commonly used in the study of prokaryotes, and has enabled a comprehensive assessment of endometrial bacterial composition, allowing identification of the full range of bacteria present in the uterine niche. In studies of healthy women, the most consistent phyla were Firmicutes, Bacteroidetes, Proteobacteria, and Actinobacteria, and the most commonly encountered genera were Lactobacillus and Streptococcus . In women with gynecological diseases, such as chronic endometritis (asymptomatic inflammation of the uterine mucosa) or endometriosis (chronic inflammatory disease caused by the presence of endometrial cells in an ectopic location) several studies have shown the presence of unusual microorganisms such as: Phyllobacterium and Sphingomonas in endometritis, Gardnerella, alpha Streptococcus, Enterococci and Escherichia coli in endometriosis.

At present, the idea of a native, optimal, healthy uterine organ microbiota that is stable over time suffers from significant limitations:

the uterine microbiota studied in healthy subjects varies widely, with little consistency among the strains found;
the populations detected may be transient, the result of an occasional invasion;
it is unclear whether the uterine microbiota changes over time or during the menstrual cycle.
However, given the significant impact of menopause on the vaginal microbiome, it is important to consider the role of hormonal fluctuations and primarily steroid therapies on the uterine microbiota during the various stages of a woman’s life and particularly in women seeking offspring, infertile, and polyabortion women.

In infertile women undergoing IVF, a dysbiotic endometrial microbiota with significant Lactobacillus deficiency has been found and some bacterial strains have also been reported to reduce the likelihood of embryo implantation , this may play a role in the mechanisms of embryo-endometrial contact, adhesion and nesting. Such alteration of the endometrial microbiota could be imputed to the therapeutic program of the assisted reproductive technique; in fact, both the induction of superovulation in the proliferative phase, prior to oocyte retrieval, and the support of the luteal phase after transfer with exogenous progesterone significantly amplify ovarian steroid hormone levels compared with the natural cycle, bringing the circulating quota well above physiological levels.

Thus it seems likely that the progressive and sudden supraphysiological increase in serum levels of 17 beta estradiol (E2) during controlled ovarian stimulation (COS), as well as the iatrogenic increase in progesterone (P) concentration resulting from its supplementation in the luteal phase, induce significant changes in the endometrial microbiota, in that particular period referred to as the “implantation window” where the relative proportion of Lactobacillus at the endometrial level is sharply decreased with a concomitant increase in pathogenic bacteria, such as Atopobium, Escherichia coli, Shigella and Prevotella creating an environmental instability capable of negatively affecting endometrial receptivity .

Other adverse effects associated with COS on the endometrial environment are mediated by a large number of genes and gene products differentially expressed during the receptive phase of the menstrual cycle . Many genes related to endometrial receptivity are regulated by hormones , and thus COS by dysregulating gene expression may impair embryo implantation according to this other pathway, compared with natural cycles without hormonal stimulation.

Understanding what happens at the time of embryo implantation has been the subject of countless studies involving researchers from a wide variety of branches: including maternal-fetal medicine, microbiology, genetics, reproductive endocrinology, and immunology, all of which have focused attention at the moment the embryo interacts with the maternal endometrium. Successful implantation requires a perfect relationship between uterus and embryo, mediated by a tightly controlled interaction between embryo and endometrium, unperturbed by external factors.

Current theories suggest that the altered microbiota may trigger an inflammatory response of the endometrium that affects the success of embryo implantation, as mediators of inflammation are tightly regulated during adhesion of the blastocyst to the mucosal wall of the endometrium .

There is growing evidence in the literature showing better pregnancy outcomes when the “freeze all” policy is adopted in IVF cycles, i.e., cryopreservation of the entire embryo cohort and subsequent transfer to a cycle with a more physiological endometrium .

Characterization of the “resident” endometrial microbiota opens up new avenues of study on embryo-endometrial binding interferents and reproductive success, for which extensive studies involving the many branches of bio-medical science are still needed.

Attention to the vaginal environment, but especially to the endouterine environment, will result in a better assessment of the female clinical picture allowing the elimination of possible pathogenic noxes that can hinder the establishment of pregnancy.

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