The uterine microbiome in recurrent pregnancy loss

A Narrative Review Discussing the Obstetric Repercussions Due to Alterations of Personalized Bacterial Sites Developed within the Vagina, Cervix, and Endometrium

BACKGROUND

The reproductive tract microbiota that evolved as an integrative component has been studied intensively in the last decade. As a result, novel research, clinical opportunities, and perspectives have been derived following the close investigation of this microecological environment. This has paved the way for an update to and improvement of the management strategies and therapeutic approaches. However, obscurities, contradictions, and controversies arise regarding the ascension route from the vagina to the endometrium via the cervix, with finality in adverse obstetric outcomes.

METHODS

Starting from these considerations, we aimed to gather all existing data and information from four major academic databases (PubMed, ISI Web of Knowledge, Scopus, and ScienceDirect) published in the last 13 years (2010-2023) using a controlled vocabulary and dedicated terminology to enhance the coverage, identification, and sorting of potentially eligible studies.

RESULTS

Despite the high number of returned entries (n = 804), only a slight percentage (2.73%) of all manuscripts were deemed eligible following two rounds of evaluation. Cumulatively, a low level of Lactobacillus spp. and of other core microbiota members is mandatory, with a possible eubiosis-to-dysbiosis transition leading to an impairment of metabolic and endocrine network homeostasis. This transposes into a change in the pro-inflammatory landscape and activation of signaling pathways due to activity exerted by the bacterial lipopolysaccharides (LPSs)/endotoxins that further reflect a high risk of miscarriage in various stages. While the presence of some pathogenic entities may be suggestive of an adverse obstetric predisposition, there are still pros and cons of the role of specific strains, as only the vagina and cervix have been targeted as opposed to the endometrium, which recently started to be viewed as the key player in the vagina-cervix-endometrium route. Consequently, based on an individual's profile, diet, and regime, antibiotics and probiotics might be practical or not.

CONCLUSIONS

Resident bacteria have a dual facet and are beneficial for women's health, but, at the same time, relaying on the abundance, richness, and evenness that are definitory indexes standing as intermediaries of a miscarriage.

Derangements of vaginal and cervical canal microbiota determined with real-time PCR in women with recurrent miscarriages

Balanced vaginal microbiota and, as a continuum, cervical canal microbiota help prevent reproductive disorders, including recurrent miscarriage (RM). In a significant proportion of couples with RM, routine diagnostic workup cannot find any manageable cause, leading to a requirement for new diagnostic tools. In the present study, we determined the quantitative composition of the microbiota of the vagina and cervical canal, assessed by real-time polymerase chain reaction, in women with RM. It also evaluated their derangements related to the pathogenesis of RM, and thus the suitability of this test as a diagnostic tool for managing RM. Vaginal and cervical canal specimens of 25 women with RM and 25 healthy volunteers were collected. The test results revealed information about the total vaginal bacterial biomass by measuring the abundance of Lactobacillus spp.; other bacteria; and pathogens, including Mycoplasma hominis, Mycoplasma genitalium, Ureaplasma (urealyticum + parvum), and Candida spp. Overall, the findings of this study implied the abundance of Lactobacillus spp. decreased in women with RM with an increase in the abundance of other microorganisms in accordance with the reduction in the abundance of Lactobacillus spp. due to aerobic vaginitis and bacterial vaginosis. Vaginal and cervical canal microbiota need to be considered during the diagnostic workup of women with RM.IMPACT STATEMENTWhat is already known on this subject? Recurrent miscarriage (RM) is a well-known reproductive disorder. Its diagnostic workup is not successful in determining the underlying problem in many cases. Hence, novel diagnostic tools based on real-time polymerase chain reaction (PCR) are needed for evaluating reproductive microbiota, which are considerably reliable, to satisfy the expectations of women with RM.What do the results of this study add? Overall, the decrease in the abundance of Lactobacillus spp. was found to be related to RM, and the patterns of the presence of other microorganisms were in accordance with the reduction in the abundance of Lactobacillus spp. These findings suggested an important role of vaginal and cervical canal microbiota in the pathogenesis of RM.What are the implications of these findings for clinical practice and/or further research? Additional research is warranted to elucidate the functional impact of altered components of the microbiota of vaginal and cervical canals on the physiology of the local cervical canal and its participation in the microbiota of the endometrial cavity, especially regarding unsuccessful pregnancies as a result of the disturbed physiology of the local endometrial microenvironment. However, possible applications of real-time PCR-based tests for the screening of subclinical infections in clinical practice require the performance of further investigations in patients with RM.