Normal flora and bacterial vaginosis in pregnancy: an overview

Myometrium infection decreases TREK1 through NHE1 and increases contraction in pregnant mice

Intrauterine infection during pregnancy can enhance uterine contractions. A two-pore K+ channel TREK1 is crucial for maintaining uterine quiescence and reducing contractility, with its properties regulated by pH changes in cell microenvironment. Meanwhile, the sodium hydrogen exchanger 1 (NHE1) plays a pivotal role in modulating cellular pH homeostasis, and its activation increases smooth muscle tension. By establishing an infected mouse model of Escherichia coli (E. coli) and lipopolysaccharide (LPS), we used Western blotting, real-time quantitative polymerase chain reaction, and immunofluorescence to detect changes of TREK1 and NHE1 expression in the myometrium, and isometric recording measured the uterus contraction. The NHE1 inhibitor cariporide was used to explore the effect of NHE1 on TREK1. Finally, cell contraction assay and siRNA transfection were performed to clarify the relationship between NHE1 and TREK1 in vitro. We found that the uterine contraction was notably enhanced in infected mice with E. coli and LPS administration. Meanwhile, TREK1 expression was reduced, whereas NHE1 expression was upregulated in infected mice. Cariporide alleviated the increased uterine contraction and promoted myometrium TREK1 expression in LPS-injected mice. Furthermore, suppression of NHE1 with siRNA transfection inhibited the contractility of uterine smooth muscle cells and activated the TREK1. Altogether, our findings indicate that infection increases the uterine contraction by downregulating myometrium TREK1 in mice, and the inhibition of TREK1 is attributed to the activation of NHE1.NEW & NOTEWORTHY Present work found that infection during pregnancy will increase myometrium contraction. Infection downregulated NHE1 and followed TREK1 expression and activation decrease in myometrium, resulting in increased myometrium contraction.

Vitamin D deficiency increases the risk of bacterial vaginosis during pregnancy: Evidence from a meta-analysis based on observational studies

Background

Bacterial vaginosis (BV) is the most common microbiological syndrome in women of childbearing age, causing numerous adverse health issues in pregnant women. Several observational studies have discussed the association between vitamin D deficiency and the risk of BV during pregnancy, but the results were inconclusive. Therefore, this meta-analysis aimed to explore the association between vitamin D deficiency and BV risk in pregnant women.

Materials and methods

We searched four databases, including PubMed, Embase, Cochrane Library, and Web of Science, from their inception to July 2022. Pooled odds ratios (OR) with corresponding 95% confidence intervals (CI) were estimated using random effects models. Additionally, we conducted subgroup analyses to identify the potential sources of between-study heterogeneity. Sensitivity analysis was performed using the method of exclusion, one study at a time. Publication bias was examined using Egger’s test and funnel plot.

Results

A total of 14 studies from 13 articles including 4,793 participants were eligible for this meta-analysis. The outcome showed that vitamin D deficiency may increase the risk of BV during pregnancy by 54% (OR, 1.54; 95% CI, 1.25–1.91; P < 0.001). In subgroup analyses, positive associations were also found in studies that were: conducted in black women (OR, 1.56; 95% CI, 0.98–2.48; P = 0.060), focused on the first trimester of pregnancy (OR, 2.22; 95% CI, 1.35–3.64; P = 0.002), of high quality (OR, 3.05; 95% CI, 1.26–7.41; P = 0.014), and adjusted for confounders (OR, 1.28; 95% CI, 1.06–1.55; P = 0.012). Sensitivity analysis reported that BV risk during pregnancy resulting from vitamin D deficiency increased by 157% (OR, 2.57; 95% CI, 1.50–4.42; P = 0.001) when removing the first two high-weight studies. Publication bias was observed using Egger’s test (t = 3.43, P = 0.005) and a visual funnel plot.

Conclusion

This meta-analysis showed that vitamin D deficiency is positively associated with the risk of BV during pregnancy. Further high-quality prospective cohort studies are needed to determine whether vitamin D intake reduces the prevalence of BV in pregnant women.

Aerobic Vaginitis Diagnosis Criteria Combining Gram Stain with Clinical Features: An Establishment and Prospective Validation Study

Wet-mount microscopy aerobic vaginitis (AV) diagnostic criteria need phase-contrast microscopy and keen microscopists, and the preservation of saline smears is less common in clinical practice. This research work developed new AV diagnostic criteria that combine Gram stain with clinical features. We enrolled 325 AV patients and 325 controls as a study population to develop new AV diagnostic criteria. Then, an independent group, which included 500 women, was used as a validation population. AV-related microscopic findings on Gram-stained and wet-mount smears from the same participants were compared. The accuracy of bacterial indicators from the two methods was verified by bacterial 16S rRNA V4 sequencing (n = 240). Logistic regression was used to analyse AV-related clinical features. The screened clinical features were combined with Gram-stain microscopic indicators to establish new AV diagnostic criteria. There were no significant differences in the leukocyte counts or the parabasal epitheliocytes (PBC) proportion between the Gram-stain and wet-mount methods (400×). Gram stain (1000×) satisfied the ability to identify bacteria as verified by 16S rRNA sequencing but failed to identify toxic leukocytes. The new criteria included: Lactobacillary grades (LBG) and background flora (Gram stain, 1000×), leukocytes count and PBC proportion (Gram stain, 400×), and clinical features (vaginal pH > 4.5, vagina hyperemia, and yellow discharge). These criteria satisfied the accuracy and reliability for AV diagnosis (Se = 86.79%, Sp = 95.97%, and Kendall’s W value = 0.899) in perspective validation. In summary, we proposed an alternative and valuable AV diagnostic criteria based on the Gram stain, which can make it possible to diagnose common vaginitis like AV, BV, VVC, and mixed infections on the same smear and can be available for artificial intelligence diagnosis in the future.

Prevalence, risk factors and adverse pregnancy outcomes of second trimester bacterial vaginosis among pregnant women in Bukavu, Democratic Republic of the Congo

BACKGROUND

Bacterial vaginosis (BV) is the most common gynecological condition in women of reproductive age and associated with adverse pregnancy outcomes. In the Democratic Republic of the Congo (DRC), neonatal mortality rate is as high as 2.8 percent with preterm birth (PTB) and low birth weight (LBW) as leading causes. Because no studies have addressed BV in DRC, we aimed to investigate the prevalence of BV, the risk factors and the association between BV and adverse pregnancy outcomes in a population of pregnant women from Bukavu, DRC.

METHODS

A total of 533 pregnant women in the second trimester of pregnancy were recruited in the Provincial Reference Hospital of Bukavu, DRC, between January and October 2017, and followed until delivery. Clinical and sociodemographic data of mother and newborn, and data on (vaginal) hygiene practices, sexual behavior and reproductive history were collected. BV was diagnosed by Nugent scoring of Gram-stained vaginal smears. Two multivariate regression models were built to identify risk factors for BV and to investigate BV as a risk factor for adverse pregnancy outcomes.

RESULTS

The prevalence of BV was 26.3% and approximately half of the women with BV were asymptomatic. Independent risk factors for BV were the use of alternatives to water for intravaginal washing, concurrent partners, unemployed status, the presence of vaginal Candida and clay consumption. BV was independently associated with both LBW and PTB of an infant with LBW.

CONCLUSION

The prevalence of BV in Bukavu is high but in line with the global average. BV was associated with adverse pregnancy outcomes in our study population. Hence, research on modifiable risk factor-based interventions to reduce the prevalence of BV, and on screening/treatment of BV during antenatal care should be explored to reduce neonatal mortality and morbidity.

Pathogenic Spectrum and Resistance Pattern of Bloodstream Infections Isolated from Postpartum Women: A Multicenter Retrospective Study

Purpose

Bloodstream infections (BSIs) cause morbidity and mortality in postpartum patients, resulting in poor prognosis for both mother and neonate. Gram-negative bacteremia is a public health threat, with high mortality among vulnerable populations and significant global economic costs. Gram-negative bacteremia and antimicrobial resistance are increasing. This study retrospectively analyzed the pathogen distribution and drug sensitivity among postpartum patients with BSIs to identify appropriate antibacterial agents for perioperative therapy.

Material and Methods

All bacteremia cases between January 2015 and December 2020 from three Health Centers for Women and Children in Chongqing, China, were retrospectively reviewed. Clinical data were collected from medical records and charts. Blood samples were cultured by BD BACTEC FX200. Bacterial and fungal species and bacterial susceptibility were identified by a BD Phoenix^TM M50 automatic detection machine.

Results

In total, 274 pathogenic strains were isolated from 272 blood samples. Excluding 25 suspected contamination strains, 248 blood samples yielded 249 microorganisms, including 214 gram-negative bacteria (85.9%), 34 gram-positive bacteria (13.6%), and 1 fungus (0.5%). Escherichia coli (E. coli) was the most frequently isolated pathogen, both overall and among gram-negative bacilli (73.5%). Streptococcus agalactiae represented 3.6% of gram-positive cocci (n = 9). Laboratory-confirmed anaerobic infections comprised 9.2% of cases (n = 23). Additionally, 47.4% of postpartum patients with BSIs suffered premature rupture of membranes (PROM), a suspected infection risk factor. Drug sensitivity levels remained unchanged for less commonly used drugs, but resistance increased against commonly used drugs. Specifically, E. coli resistance against fourth-generation cephalosporins increased during this study period.

Conclusion

E. coli is the most common gram-negative bacillus in postpartum patients with BSIs, and increased anaerobic bacterial detections suggest genital tract inflammation control before delivery is necessary. Effective drug resistance monitoring remains necessary to alleviate bacterial resistance, such as preventing inappropriate antibiotic applications.

The Association Between Vaginal Microbiota Dysbiosis, Bacterial Vaginosis, and Aerobic Vaginitis, and Adverse Pregnancy Outcomes of Women Living in Sub-Saharan Africa: A Systematic Review

Background: Previous studies have described the association between dysbiosis of the vaginal microbiota (VMB) and related dysbiotic conditions, such as bacterial vaginosis (BV) and aerobic vaginitis (AV), and various adverse pregnancy outcomes. There is limited overview of this association from countries in sub-Saharan Africa (SSA), which bear a disproportionally high burden of both vaginal dysbiotic conditions and adverse pregnancy outcomes. This systematic review assesses the evidence on the association between VMB dysbiosis, BV, and AV, and late adverse pregnancy outcomes in women living in SSA.

Methods: The Preferred Reporting Items for Systematic Review and Meta-Analysis Statement (PRISMA) guidelines were followed. Three databases [PubMed, Embase (Ovid), and Cochrane] were used to retrieve observational and intervention studies conducted in SSA that associated VMB dysbiosis, BV, or AV and preterm birth/labor/delivery, preterm rupture of membranes (PROM), low birthweight, small for gestational age, intrauterine growth restriction, intrauterine infection, intrauterine (fetal) death, stillbirth, perinatal death, or perinatal mortality.

Results: Twelve studies out of 693 search records from five SSA countries were included. One study identified a positive association between VMB dysbiosis and low birthweight. Despite considerable differences in study design and outcome reporting, studies reported an association between BV and preterm birth (7/9), low birthweight (2/6), PROM (2/4), intrauterine infections (1/1), and small for gestational age (1/1). None of the retrieved studies found an association between BV and pregnancy loss (5/5) or intrauterine growth retardation (1/1). At least two studies support the association between BV and PROM, low birthweight, and preterm birth in Nigerian pregnant women. No reports were identified investigating the association between AV and late adverse pregnancy outcomes in SSA.

Conclusion: Two of the included studies from SSA support the association between BV and PROM. The remaining studies show discrepancies in supporting an association between BV and preterm birth or low birthweight. None of the studies found an association between BV and pregnancy loss. As for the role of VMB dysbiosis, BV, and AV during pregnancy among SSA women, additional research is needed. These results provide useful evidence for prevention efforts to decrease vaginal dysbiosis and its contribution to adverse pregnancy outcomes in SSA.

The Interplay Between Reproductive Tract Microbiota and Immunological System in Human Reproduction

In the last decade, the microbiota, i.e., combined populations of microorganisms living inside and on the surface of the human body, has increasingly attracted attention of researchers in the medical field. Indeed, since the completion of the Human Microbiome Project, insight and interest in the role of microbiota in health and disease, also through study of its combined genomes, the microbiome, has been steadily expanding. One less explored field of microbiome research has been the female reproductive tract. Research mainly from the past decade suggests that microbial communities residing in the reproductive tract represent a large proportion of the female microbial network and appear to be involved in reproductive failure and pregnancy complications. Microbiome research is facing technological and methodological challenges, as detection techniques and analysis methods are far from being standardized. A further hurdle is understanding the complex host-microbiota interaction and the confounding effect of a multitude of constitutional and environmental factors. A key regulator of this interaction is the maternal immune system that, during the peri-conceptional stage and even more so during pregnancy, undergoes considerable modulation. This review aims to summarize the current literature on reproductive tract microbiota describing the composition of microbiota in different anatomical locations (vagina, cervix, endometrium, and placenta). We also discuss putative mechanisms of interaction between such microbial communities and various aspects of the immune system, with a focus on the characteristic immunological changes during normal pregnancy. Furthermore, we discuss how abnormal microbiota composition, “dysbiosis,” is linked to a spectrum of clinical disorders related to the female reproductive system and how the maternal immune system is involved. Finally, based on the data presented in this review, the future perspectives in diagnostic approaches, research directions and therapeutic opportunities are explored.

Prevalence of vulvovaginal candidiasis, bacterial vaginosis and trichomoniasis in pregnant women attending antenatal clinic in the middle belt of Ghana

Background

Vaginal infections usually caused by Candida sp, organisms responsible for bacterial vaginosis and Trichomonas vaginalis are associated with considerable discomfort and adverse outcomes during pregnancy and child birth. The study determined the prevalence of vulvovaginal candidiasis (VVC), bacterial vaginosis (BV) and trichomoniasis (TV) in pregnant women attending antenatal clinic at the Kintampo Municipal Hospital.

Methods

A study adopted a cross sectional design and recruited 589 pregnant women after seeking their informed consent from September, 2014 to March, 2015. Semi-structured questionnaire were administered to participants and vaginal swabs were collected. The samples were analysed using wet mount method and Gram stain (Nugent criteria) for vaginal infection. Univariate and multivariate analysis were used to investigate association of risk factors to vaginal infections.

Results

The overall prevalence of at least one vaginal infection was 56.4%. The prevalence of vulvovaginal candidiasis, bacterial vaginosis and trichomoniasis were 36.5, 30.9 and 1.4% respectively. Women with more than four previous pregnancies (OR: 0.27, 95% CI: 0.13–0.58) and those in the third trimester of pregnancy (OR: 0.54, CI: 0.30–0.96) were associated with a lower risk of bacterial vaginosis. Douching and antibiotic use were neither associated with VVC or BV.

Conclusion

The prevalence of vaginal infections was high among pregnant women in the Kintampo area. There is the need for interventions such as adequate investigations and early treatment of vaginal infections to reduce the disease burden to avoid associated complications.

Prevalence of bacterial vaginosis in pregnancy in a tertiary health institution, south western Nigeria

Introduction

Bacterial vaginosis (BV) is the most common cause of vaginal discharge in women of child bearing age. Bacterial vaginosis has emerged as a global health issue due to the adverse outcome in pregnancy and in the puerperium. The study determined the prevalence of BV and outcome of delivery among pregnant women.

Methods

Socio-demographic data and vaginal swab samples were obtained from 362 consecutive pregnant women with abnormal vaginal discharge attending antenatal clinic in Ekiti State University Teaching Hospital, Ado-Ekiti. Data were analysed using SPSS statistical software 21 and association between variables was compared using Chi square.

Results

The prevalence of BV among pregnant women with abnormal vaginal discharge in this study was 16.6%. Age group 25-34yrs, multiparity and higher education were significantly associated with BV, p < 0.05. Symptoms such as vulvar itching, dyspareunia, lower abdominal pains and characteristic of vaginal discharge such as colour and consistency were significantly associated with BV, p < 0.05. Women with bacterial vaginosis significantly had prelabour rupture of fetal membrane and their babies were born prematurely with low birth weight and Apgar score of less than 5 at one minute, p < 0.05. However, there was no difference statistically in rate of admission into special care baby unit among the women, p > 0.05.

Conclusion

The findings of this study suggest that there should be screening for BV in pregnant women presenting with abnormal vaginal discharge so that they could be treated accordingly. This will mitigate the complications arising from bacterial vaginosis.

Determination of Antibacterial and Technological Properties of Vaginal Lactobacilli for Their Potential Application in Dairy Products

Functional foods could differently affect human health in relation to the gender. Recent studies have highlighted the anti-Candida and anti-Chlamydia activities of some Lactobacillus strains isolated from the vagina of healthy women. Considering these important beneficial activities on women's health, the preparation of functional food containing active vaginal lactobacilli can represent a great scientific challenge for the female gender. In this context, the aim of this work was to study some functional and technological properties of 17 vaginal strains belonging to the species Lactobacillus crispatus, Lactobacillus gasseri, and Lactobacillus vaginalis in the perspective to include them in dairy products. The antagonistic activities against the pathogenic and spoilage species associated to food products and against the principal etiological agents of the genitourinary tract infections were evaluated. Moreover, the vaginal lactobacilli were characterized for their antibiotic resistance, and for their fermentation kinetics and viability during the refrigerated storage in milk. Finally, the volatile molecule profiles of the obtained fermented milks were determined. The results showed that several strains, mainly belonging to the species Lactobacillus crispatus, exhibited a significant antagonistic activity against spoilage and pathogenic microorganisms of food interest, as well as against urogenital pathogens. All the vaginal lactobacilli showed antimicrobial activity against strains belonging to the foodborne pathogenic species Listeria monocytogenes, Listeria innocua, Eenterococcus faecalis and Escherichia coli. In addition, most of the Lactobacillus strains were active toward the main pathogens responsible of vaginal and urinary tract infections including Staphylococcus aureus, Enterococcus faecium, Gardnerella vaginalis, and Proteus mirabilis. The antimicrobial activity can be attributed to the high production of organic acids. The fermentation kinetics in milk indicated the unsuitability of these lactobacilli as fermentation starters for the industrial production of dairy products. However, some strains, belonging to the species Lactobacillus crispatus and Lactobacillus gasseri, maintained a high viability in pasteurized milk at 4°C for over a month, showing their potential application as adjunct cultures for the production of female gender foods. These data represent a first step for the set-up of a new functional dairy product, directed to the women well-being, contributing also to innovate the dairy sector.

Lactobacilli Dominance and Vaginal pH: Why Is the Human Vaginal Microbiome Unique?

The human vaginal microbiome is dominated by bacteria from the genus Lactobacillus, which create an acidic environment thought to protect women against sexually transmitted pathogens and opportunistic infections. Strikingly, lactobacilli dominance appears to be unique to humans; while the relative abundance of lactobacilli in the human vagina is typically >70%, in other mammals lactobacilli rarely comprise more than 1% of vaginal microbiota. Several hypotheses have been proposed to explain humans' unique vaginal microbiota, including humans' distinct reproductive physiology, high risk of STDs, and high risk of microbial complications linked to pregnancy and birth. Here, we test these hypotheses using comparative data on vaginal pH and the relative abundance of lactobacilli in 26 mammalian species and 50 studies (N = 21 mammals for pH and 14 mammals for lactobacilli relative abundance). We found that non-human mammals, like humans, exhibit the lowest vaginal pH during the period of highest estrogen. However, the vaginal pH of non-human mammals is never as low as is typical for humans (median vaginal pH in humans = 4.5; range of pH across all 21 non-human mammals = 5.4-7.8). Contrary to disease and obstetric risk hypotheses, we found no significant relationship between vaginal pH or lactobacilli relative abundance and multiple metrics of STD or birth injury risk (P-values ranged from 0.13 to 0.99). Given the lack of evidence for these hypotheses, we discuss two alternative explanations: the common function hypothesis and a novel hypothesis related to the diet of agricultural humans. Specifically, with regard to diet we propose that high levels of starch in human diets have led to increased levels of glycogen in the vaginal tract, which, in turn, promotes the proliferation of lactobacilli. If true, human diet may have paved the way for a novel, protective microbiome in human vaginal tracts. Overall, our results highlight the need for continuing research on non-human vaginal microbial communities and the importance of investigating both the physiological mechanisms and the broad evolutionary processes underlying human lactobacilli dominance.

Abnormal vaginal colonization by gram-negative bacteria is significantly higher in pregnancy conceived through infertility treatment compared to natural pregnancy

OBJECTIVE

To compare abnormal vaginal colonization between natural pregnancy and pregnancy by infertility treatment in high-risk parturient women and to examine the association between abnormal vaginal colonization and early-onset neonatal sepsis (EONS).

METHODS

The clinical characteristics, vaginal culture result, and delivery outcome of patients who admitted to our high-risk unit between 2005 and 2014 were retrospectively reviewed and compared. We investigated the prevalence of EONS according to maternal vaginal colonization and examined the concordance between maternal vaginal bacteria and etiologic microorganism causing EONS.

RESULTS

Among 1096 pregnancies, the rate of vaginal colonization by gram-negative bacteria, especially Escherichia coli was significantly higher in pregnancies by infertility treatment after adjustment of confounding variables (E. coli, OR [95% CI]: 2.47 [1.33-4.57], p = 0.004). The rate of EONS was significantly higher in neonates with maternal abnormal vaginal bacteria colonization (OR [95% CI]: 3.38 [1.44-7.93], p = 0.005) after adjusting for confounding variables. Notably, among microorganisms isolated from maternal vagina, E. coli and Staphylococcus aureus were consistent with the results from neonatal blood culture in EONS.

CONCLUSIONS

Our data implicate a possible association between gram-negative bacteria colonization and infertility treatment and suggest that maternal vaginal colonization may be associated with EONS of neonates in high-risk pregnancy.

The very low birth weight infant microbiome and childhood health

This review describes current understandings about the nature of the very low birth weight infant (VLBW) gut microbiome. VLBW infants often experience disruptive pregnancies and births, and prenatal factors can influence the maturity of the gut and immune system, and disturb microbial balance and succession. Many VLBWs experience rapid vaginal or Caesarean births. After birth these infants often have delays in enteral feeding, and many receive little or no mother's own milk. Furthermore the stressors of neonatal life in the hospital environment, common use of antibiotics, invasive procedures and maternal separation can contribute to dysbiosis. These infants experience gastrointestinal dysfunction, sepsis, transfusions, necrotizing enterocolitis, oxygen toxicity, and other pathophysiological conditions that affect the normal microbiota. The skin is susceptible to dysbiosis, due to its fragility and contact with NICU organisms. Dysbiosis in early life may resolve but little is known about the timing of the development of the signature gut microbiome in VLBWs. Dysbiosis has been associated with a number of physical and behavioral problems, including autism spectrum disorders, allergy and asthma, gastrointestinal disease, obesity, depression, and anxiety. Dysbiosis may be prevented or ameliorated in part by prenatal care, breast milk feeding, skin to skin contact, use of antibiotics only when necessary, and vigilance during infancy and early childhood.

Use of locally delivered dequalinium chloride in the treatment of vaginal infections: a review

Background

Vaginal infections are responsible for a large proportion of gynaecological outpatient visits. Those are bacterial vaginosis (BV), vulvovaginal candidosis (VVC), aerobic vaginitis (AV) associated with aerobic bacteria, and mixed infections. Usual treatments show similar acceptable short-term efficacy, but frequent recurrences and increasing microbial resistance are unsolved issues. Furthermore, vaginal infections are associated with a variety of serious adverse outcomes in pregnancy and generally have a major impact on quality of life. Identifying the correct therapy can be challenging for the clinician, particularly in mixed infections.

Findings

Dequalinium chloride (DQC) is an anti-microbial antiseptic agent with a broad bactericidal and fungicidal activity. Systemic absorption after vaginal application of DQC is very low and systemic effects negligible. Vaginal DQC (Fluomizin^®vaginal tablets) has been shown to have equal clinical efficacy as clindamycin in the treatment of BV. Its broad antimicrobial activity makes it appropriate for the treatment of mixed vaginal infections and in case of uncertain diagnosis. Moreover, resistance of pathogens is unlikely due to its multiple mode of action, and vaginal DQC provides also a reduced risk for post-treatment vaginal infections.

Conclusions

Vaginal DQC (10 mg) as 6-day therapy offers a safe and effective option for empiric therapy of different vaginal infections in daily practice. This review summarizes the available and relevant pharmacological and clinical data for the therapy of vaginal infections with vaginal DQC and provides the rationale for its use in daily gynaecologic practice.

BV and non-BV associated Gardnerella vaginalis establish similar synergistic interactions with other BV-associated microorganisms in dual-species biofilms

Dual-species biofilm formation between Gardnerella vaginalis strains isolated from women with or without bacterial vaginosis (BV) and other 24 BV-associated microorganisms support that the key difference in virulence potential between BV-negative and BV-positive G. vaginalis strains seems not to be related with biofilm maturation.