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Microbiome analysis flowchart
The uterine microbiome is the commensal, nonpathogenic, bacteria, viruses, yeasts/fungi present in a healthy uterus, amniotic fluid and endometrium and the specific environment which they inhabit. It has been only recently confirmed that the uterus and its tissues are not sterile.[1] Due to improved 16S rRNA genesequencing techniques, detection of bacteria that are present in low numbers is possible.[2]
Using this procedure that allows the detection of bacteria that cannot be cultured outside the body, studies of microbiota present in the uterus are expected to increase.[3]
Uterine microbiome and fertility
In the past, the uterine cavity had been traditionally considered to be sterile, but potentially susceptible to be affected by vaginal bacteria. However, this idea has been disproved. Moreover, it's been shown that endometrial and vaginal microbiota can differ in structure and composition in some women.
The microbiome of the innermost layer of the uterus, the endometrium, may influence its capacity to allow an embryo to implant. The existence of more than 10% of non-Lactobacillus bacteria in the endometrium is correlated with negative impacts on reproductive function and should be considered as an emerging cause of implantation failure and pregnancy loss.[4]
Characteristics
Bacteria, viruses and one genus of yeasts are a normal part of the uterus before and during pregnancy.[5] The uterus has been found to possess its own characteristic microbiome that differs significantly from the vaginal microbiome, consisting primarily of lactobacillus species, and at far fewer numbers.[6] In addition, the immune system is able to differentiate between those bacteria normally found in the uterus and those that are pathogenic. Hormonal changes have an effect on the microbiota of the uterus.[7]
The organisms listed below have been identified as commensals in the healthy uterus. Some also have the potential for growing to the point of causing disease:
Prophylactic antibiotics have been injected into the uterus to treat infertility. This has been done before the transfer of embryos with the intent to improve implantation rates. No association exists between successful implantation and antibiotic treatment.[11] Infertility treatments often progress to the point where a microbiological analysis of the uterine microbiota is performed. Preterm birth is associated with certain species of bacteria that are not normally part of the healthy uterine microbiome.[5]
Immune response
The immune response becomes more pronounced when bacteria are found that are not commensal.[5]
History
Investigations into reproductive-associated microbiomes began around 1885 by Theodor Escherich. He wrote that meconium from the newborn was free of bacteria. There was a general consensus at the time and even recently that the uterus was sterile and this was referred to as the sterile womb paradigm. Other investigations used sterile diapers for meconium collection. No bacteria were able to be cultured from the samples. Other studies showed that bacteria were detected and were directly proportional to the time between birth and the passage of meconium.[1]
Research
Investigations into the role of the uterine microbiome in the development of the infant microbiome are ongoing.[1]
^Moreno I, Codoñer FM, Vilella F, Valbuena D, Martinez-Blanch JF, Jimenez-Almazán J, Alonso R, Alamá P, Remohí J, Pellicer A, Ramon D, Simon C. Evidence that the endometrial microbiota has an effect on implantation success or failure. Am J Obstet Gynecol. 2016 Dec;215(6):684-703. doi: 10.1016/j.ajog.2016.09.075.
