Effect of Reproductive System Dysbiosis on the Course of Pregnancy

The vaginal microbiota is dominated by bacteria of the genus Lactobacillus and low bacterial diversity. These bacteria produce lactic acid which makes it unfavorable for the growth of pathogens, and the production of hydrogen peroxide and bacteriocins further enhances this effect (Greenbaum et al. 2018; Taddei et al. 2018). In addition, the vaginal microbiota is distinguished by variability and instability. As a result, it is difficult to unequivocally define the composition of normal vaginal microbiota for all women, especially given that even for a single woman the composition can change over time (Greenbaum et al. 2018). The composition of vaginal microbiota fluctuates under the influence of many factors (Kervinen et al. 2019).

During pregnancy, some of these factors change significantly for e.g. hormone value, resulting in changes in microbiota composition during this period of a woman’s life. Particularly noteworthy are hormonal changes. For this reason, in the vaginal microenvironment during pregnancy, bacteria of the genus Lactobacillus gain an even greater advantage than before pregnancy (Taddei et al. 2018; Heil et al. 2019; Serrano et al. 2019). Many factors contribute to the stabilization of vaginal microbiota during pregnancy: the absence of cyclic hormonal fluctuations (Walther-António et al. 2014).

The multiple changes that occur in a woman’s body during pregnancy affect the state of microbiota of the reproductive system. The vaginal microbiota during pregnancy is characterized by less variability and diversity than the vaginal microbiota during the non-pregnancy period. This leads to an increased degree of stability in the pregnancy microbiota, which can reduce the risk of infections and complications during pregnancy. The diversity of pregnancy microbiota is highest during the first trimester of pregnancy and decreases during the second and third trimesters of pregnancy, as estrogen levels increase in a woman’s body. However, it is likely that by the final stage of pregnancy, the degree of diversity of the vaginal microbiota begins to increase again. The degree of protection against the growth of pathogenic bacteria depends on the species and strain of Lactobacillus. Lactobacillus crispatus is more characteristic of a normal vaginal microbial state than Lactobacillus iners, which is more prone to transition of the vaginal microbiota to an abnormal state, increasing the risk of developing bacterial vaginosis. It appears that several species of Lactobacillus may predominate during pregnancy (Nuriel-Ohayon et al. 2016; Greenbaum et al. 2018; Kervinen et al. 2019; Mei et al. 2019; Bagga and Arora 2020; Gupta et al. 2020; Pacha-Herrera et al. 2020; Rasmussen et al. 2020). During pregnancy, α-diversity and β-diversity are also reported to decrease (Nuriel-Ohayon et al. 2016; Schoenmakers et al. 2019). Interestingly, at the end of pregnancy, the gestational microbiota becomes similar again to the pre-pregnancy microbiota (Kervinen et al. 2019). Also, other studies confirm that there is an increase in the diversity of vaginal microbiota at the end of pregnancy (Rasmussen et al. 2020).

The role of L. crispatus and L. iners in pregnancy has been confirmed in many studies (Mei et al. 2019; Mls et al. 2019; Zheng et al. 2019). For example, the study conducted by Zheng et al. showed that the amount of L. iners was decreased in the second and third trimesters of pregnancy. In addition, markers of vaginal inflammation, such as the degree of vaginal purity and leukocyte esterase activity, increased as the number of L. iners increased (Zheng et al. 2019). In contrast, Serrano et al. conducted a study on women of African, Hispanic, and European descent who were and were not pregnant. During pregnancy vaginal microbiota shifted toward lactobacilli. During pregnancy, a decrease in the diversity of bacterial microbiota and a strengthening of the predominance of Lactobacillus in the vaginal microenvironment was observed. The decrease in the vagina in the number of such bacteria as Gardnerella vaginalis, Atopobium vaginae, Sneathia amnii, and others results in a decrease in susceptibility to infection with diseases, sexually transmitted and the risk of developing bacterial vaginitis (Serrano et al. 2019).

Experiments by Walter-Antonio et al. showed that the dominant vaginal species is L. crispatus. The dominant profile in L. crispatus is associated with lower diversity than in L. iners, suggesting greater dominance of L. crispatus than L. iners. L. iners may suggest greater susceptibility to dysbiosis than L. crispatus (Walther-António et al. 2014). The relationship between the composition of vaginal microbiota and ethnicity has also been shown in studies by other authors (Aagaard et al. 2012; Freitas et al. 2017; Nuriel-Ohayon et al. 2016).

Recent studies indicate that the uterus has its own specific microbiota. It is not easy to detect the physiological microbiota inhabiting the endometrium (uterine mucosa). Chen et al. reported that uterine microbiota is dominated by Lactobacillus (30.6%) and other microorganisms such as Pseudomonas, Acinetobacter, and Vagococcus (Chen et al. 2017). Koedooder et al. in suggested that uterine microbiota is dominant by families Lactobacillaceae, Streptococaceae, and Bifidobacteriaceae families (Koedooder et al. 2019).

The microbiological composition of the endometrium during pregnancy is of particular interest. However, the method of obtaining such material for study is problematic. As a result of such an experiment many genus were identified in the endometrium of normal pregnancy: Cutibacterium, Escherichia, Staphylococcus, Acinetobacter, Streptococcus, Corynebacterium. Bacteria of the genus Lactobacillus showed high variability in presence in the samples tested. The authors suggest that although a uterine microbiota with a high density of Lactobacillus is conducive to achieving pregnancy, the presence of these bacteria during pregnancy does not appear to be a prerequisite for a normal pregnancy (Leoni et al. 2019). Moreno et al. prepared a case report about women who had spontaneous miscarriages and the next physiological pregnancy. They collected data about uterine microbiota in both cases. In pregnancy that ended miscarriage, the uterine fluid included lactobacilli and appeared to have greater diversity than in physiological pregnancy (Moreno et al. 2020).

The use of modern molecular techniques to identify bacteria showed the presence of bacteria in the placenta. 16S rRNA sequencing allowed the determination of the placental microbial composition (Olaniy et al. 2020). Using the sequencing method, the placental microbiome was characterized in more than 300 healthy, carried pregnancies and in pregnancies that terminated prematurely (Aagaard et al. 2012). The microbiome of isolated placental tissue included such microorganisms as Escherichia coli, Cutibacterium acnes, Bacteroides sp., L. crispatus, and L. iners (Aagaard et al. 2014). Gomez de Agüero et al. identified in placenta C. acnes, Enterobacteriaceae sp., and Lactobacillus (Gomez de Agüero et al. 2016). The movement of bacteria from the intestinal epithelium and oral mucosa through the maternal circulation allows a small number of bacteria to populate the placenta (Olanyiy et al. 2020). The microbiota of amniotic fluid is like that of the placenta (Schoenmakers et al. 2019).

As in the case of the placenta, the microbial composition of cord blood has not yet been clearly determined. Jimenez et al. identified umbilical cord blood bacteria. They detected bacteria such as Streptococcus sanguinis, Enterococcus faecium, and Staphylococcus epidermidis. These species occur naturally in healthy infants and are considered commensals (Jiménez et al. 2005). Tang et al. showed that cord blood microbiota was identified in 15 samples in women with gestational diabetes mellitus. Cord blood microbiota was dominated by Firmicutes, Actinobacteria, Ruminococcaceae, and Rhodococcus (Tang et al. 2020) (Fig. 1).

Miscarriage is the most common obstetric complication and 50% of all miscarriages can be caused by chromosomal aberrations, and the causes of the remaining cases remain unclear (Larsen et al. 2013). Physiological pregnancy is dominated by Lactobacillus species and low bacterial biodiversity. The beginning of pregnancy is sometimes correlated with a decreased number of lactobacilli in the vagina, and this decrease may precede miscarriage (Al-Memar et al. 2020).

Miscarriage not caused by chromosomal aberrations can be associated with a decrease in the number of Lactobacillus bacteria compared to aberrant and normal pregnancies. Miscarriage may be due to the mother’s inflammatory response to vaginal dysbiosis, which is often caused by a decrease in Lactobacillus (Grewal et al. 2020). Xu et al. showed that 56% of women who experienced an embryonic miscarriage showed a small amount of lactobacilli in a vaginal swab, while 88% of women in the control group showed a large amount of these bacteria. The group of women who experienced an embryonic miscarriage showed a higher level of diversity in the vaginal microbiota (Xu et al. 2020).

Grewal et al. reported that a reduction in the number of lactic acid bacilli in the vagina is associated with an increase in local expression of pro-inflammatory cytokines (Grewal et al. 2020). An increase in IL-2 levels and a decrease in IL-10 levels have been shown in a group of women who have experienced an embryonic miscarriage (Marzi et al. 1996; Wilczynski et al. 2005; Xu et al. 2020). In addition, a correlation has been shown between infection, the inherence of G. vaginalis, and increased levels of peripheral NK cells (pNK) (Kuon et al. 2017). The findings may indicate a relationship between vaginal microbiota, local inflammation, changes in immune parameters, and the risk of miscarriage (Kuon et al. 2017; Villa et al. 2020).

Disturbed vaginal microbiome is a risk factor for miscarriage. This factor can be modified by preventive measures (administration of prebiotics and probiotics) or therapeutic measures (antibiotic therapy) (Al-Memar et al. 2020).

Preterm labor (PTB) is a major obstetric problem and causes significant neonatal mortality (Parry et al. 1998; Bayar et al. 2020). Amniotic membrane rupture can lead to microorganisms from the vagina toward the uterus and fetus, and pathogenic bacteria can initiate the development of infection and inflammation. Infection can be both a cause and a consequence of the rupture of the fetal membranes (Bayar et al. 2020).

Before PTB occurs leukocyte activation, increases levels of pro-inflammatory cytokines and chemokines. PTB is often associated with a vaginal infection. It is suggested that matrix metalloproteinase 8 (MMP-8) changes cervical integrity and facilitates bacterial movement (Linhares et al. 2019). The loss of Lactobacillus species from the vaginal microbiota and the overgrowth of other bacteria contribute to the development of bacterial vaginitis (BV) and aerobic vaginitis (AV) (Donders et al. 2011). Elevated levels of D-lactic acid and the ratio of D-lactic acid to L-lactic acid influence the increase in extracellular matrix metalloproteinase inducer (EMM-PRIN) and MMP-8 concentrations (Witkin et al. 2013). L. crispatus can inhibit EMMPRIN activation, thus preventing infection and preterm labor. L. crispatus has immunomodulatory functions (Witkin et al. 2013).

It was proven that 40% of samples taken from women who gave birth prematurely did not contain any Lactobacillus species, while all women with birth at term consisted of at least one or more Lactobacillus species. The most common species isolated in women who gave birth at term was L. crispatus (46%), followed by L. jensenii (25%) and L. gasseri (19%). Moreover, data suggest that the presence of L. iners was one of the causes of PTB (Aslam et al. 2020). Feehily et al. identified S. amnii and Prevotella amnii species as risk factors for preterm labor (Feehily et al. 2020). The results of other studies show that normal vaginal microbiota is associated with a 75% lower risk of PTB. The authors conclude that the absence of lactobacilli combined with a higher level of anaerobic bacteria is a stronger predictor of PTB (Kosti et al. 2020). These studies show that Lactobacillus protects against PTB (Abdelmaksoud et al. 2016; Stout et al. 2017; Di Simone et al. 2020).

Dysbiosis of the vaginal microbiota has also been associated with the production of other metabolites by microorganisms, which can cause PTB. Under-representation of Lactobacillus and consequently low lactate levels can also promote abnormal pregnancy (Fettweis et al. 2014; 2019). For example, infection of cervical and vaginal epithelial cells by Ureaplasma urealyticum stimulates ammonia production and induces increased IL-8 production, which can lead to much higher cytotoxicity. On the contrary, L. crispatus appears to protect against inflammation and HeLa cell death by producing more D-lactate and less IL-8 (Cavanagh et al. 2020). As mentioned, the transition of microbiota composition from Lactobacillus spp. to Prevotella causes vaginal dysbiosis and production of pathogenic microbiota metabolites. For example, high levels of acetate and low levels of succinate, immunomodulatory relationships, have been associated with the occurrence of PTB. This is possible through higher pH and an increase in pro-inflammatory cytokines (Li et al. 2010; Mirmonsef et al. 2012; Amabebe et al. 2016; Stafford et al. 2017, Ansari et al. 2020).

Levels of vaginal inflammatory C-X-C Motif Chemokine Ligand 10 (CXCL10) and PTB were associated with the ratio of L. crispatus/L. iners, indicating possible predictive markers of PTB: cytokine levels/ Lactobacillus number. Many women give birth at term despite reduced bacterial counts of Lactobacillus species. The immune factor can modulate PTB risk regardless of Lactobacillus species (Fettweis et al. 2014; Elovitz et al. 2019; Di Simone et al. 2020).

Preeclampsia (PE) is characterized by hypertension and proteinuria and is life-threatening for the pregnant woman and her baby (Goel et al. 2015; Rana et al. 2019; Olaniyi et al. 2020]. There are many hypotheses regarding the causes of the onset of preeclampsia, including abnormalities in the development of the placenta, disturbances in the immune mechanisms between the fetus and mother, or abnormalities in the factors responsible for vasoconstriction. These changes lead to the hypertension and multiple organ failure seen in preeclampsia syndrome (Goel et al. 2015; Rana et al. 2019; Olaniyi et al. 2020). The presence of bacteria in the placenta can also affect the activity of anti-angiogenic factors and pro-angiogenic factors (Olaniyi et al. 2020).

The placental microbiome during physiological pregnancy is dominated by Lactobacillus and Grampositive and Gram-negative bacteria (Gomez et al. 2016). Dysbiosis of the placental microbiota, including an increase in the number of Bacteroides and a decrease in the number of Lactobacillus, alters the host immune response which can initiate the onset of various pregnancy complications, including preeclampsia and preterm labor (Gomez et al. 2016; Bardos et al. 2019; Olaniyi et al. 2020).

The placental microbiome has a regulatory effect on the metabolic and immune functions of the host (Olaniyi et al. 2020). Dysbiosis can change placental endothelial function and placental hypoxia and ischemia (Amarasekara et al. 2015). In addition, lipopolysaccharide can play an important role in the development of preeclampsia. Bacterial dysbiosis of the placenta can also disrupt tryptophan and fatty acid metabolism, resulting in impaired maternal and fetal energy homeostasis, which can exacerbate the course of preeclampsia (Olaniyi et al. 2020).

Local dysbiosis, causing activation of inflammatory cells and changes in the uterine myometrium, may be associated with the development of postpartum hemorrhage (PPH) (Farhana et al. 2015). After childbirth, sometimes occur a phenomenon of inability of the uterine myometrial fibers to contract. In many cases, this phenomenon may be secondary to local dysbiosis. This can promote the activation of the complement system, neutrophils, and macrophages, as well as mast cell degranulation in uterine and placental tissues, resulting in impaired uterine contractility. Indeed, an increase in the number of inflammatory cells, such as neutrophils, macrophages, and mast cells, and activation of the complement system have been observed in uterine and placental tissues in women suffering from PPH of unclear etiology (Escobar et al. 2020). These cells produce chemical mediators that affect the vascular myometrium and uterus. The exudate produced by the ongoing inflammatory reaction leads to edema and ultimately impairs uterine contractility (Farhana et al. 2015; Escobar et al. 2020). In summary, dysbiosis of the reproductive system can cause the development of a local inflammatory response, resulting in impaired uterine contractility and an increased risk of PPH.