Abstract

Introduction

The cervicovaginal ecosystem is made up of diverse microorganisms coexisting in a dynamic balance and establishing complex connections with each other and with the host. In healthy reproductive-aged women, the vaginal microbiome, generally, shows a predominance of Lactobacillus genus, and most women display the prevalence of one species among L. crispatus, L. iners, L. jensenii and L. gasseri¹. Lactobacilli promote the maintenance of the vaginal homeostasis and prevent the colonization and growth of adverse microorganisms, including those responsible for sexually transmitted infections (STIs). Such defensive function is exerted through various mechanisms, such as vaginal pH lowering, bioactive compounds production, competition for nutrients and adhesion sites, and modulation of host immune response^2–5. However, the composition of the vaginal microbiome can vary throughout a woman’s life in response to endogenous and exogenous factors, such as age, pregnancy, pharmaceutical treatments, and urogenital infections.

Urogenital infections and dysbiosis afflict over 1 billion women each year, compromising their wellbeing and affecting their reproductive health⁶. The most common vaginal dysbiosis worldwide is bacterial vaginosis (BV), which is characterized by a shift in the microbial composition from the normally Lactobacillus-dominated to a high-complexity, polymicrobial community. It has been largely established that BV condition is characterized by the presence of anaerobic bacteria, such as Gardnerella vaginalis, Atopobium spp., Prevotella spp., and high concentrations of various biogenic amines (putrescine, cadaverine and trimethylamine), short chain fatty acids (especially acetate and succinate) and low levels of some amino acids (tyrosine, glutamate)^7–9. In several epidemiological studies it has been reported that BV represents a risk factor for the acquisition of STIs^10,11.

Among infections, vulvovaginal candidiasis (VVC) affects about 75% of reproductive-aged women at least once during their lives, with about 5% of them suffering of recurrences¹². VVC is caused by Candida spp. which, in particular conditions, instead of being part of the normal vaginal microflora, become a robust opportunistic fungal pathogen, with a tendency to overgrow¹³. C. albicans is responsible for 80–92% of VVC cases. The causes and phases of the transition from the homeostasis to the pathogenic state of VVC are not clear yet, as well as the impact, if any, on the vaginal microbiome structure and metabolomic profile.

Chlamydia trachomatis (CT) represents the most common bacterial STI worldwide, and new infections probably exceed 131 million per year¹⁴. CT is a Gram-negative, obligate intracellular bacterial pathogen with a unique biphasic developmental cycle¹⁵. CT serovars from D to K are responsible for common and frequently asymptomatic urogenital infections (i.e. urethritis and cervicitis), that may lead to several sequelae and complications, including pelvic inflammatory disease (PID), ectopic pregnancy, and infertility. Globally, serovars E, F, D, and G represent the most common serovars, accounting for 60–80% of cases^16,17. Given its high prevalence and sociological impact, correlation of CT infection with the cervico-vaginal microbiome has recently gained particular attention: some cross-sectional studies have been published^18,19, and vaginal microbiome/metabolome fingerprints have been explored²⁰.

In this work, we depicted and compared the alterations that occur in the vaginal microbiome and metabolome during common genital infections. Two high-performing techniques, i.e. next generation sequencing (NGS) and proton-based nuclear magnetic resonance (¹H-NMR) spectroscopy, were applied to unravel the vaginal microbiome composition and the metabolome fingerprint, respectively, highlighting common trends and correlations. The relatively-less characterized ecosystems corresponding to VVC and CT infection were compared to the healthy status (reference of eubiosis) and BV condition (reference of dysbiosis).

Results

Study group

A total of 79 patients were included in the study: specifically, 21 were considered healthy (HC), 20 received a diagnosis of bacterial vaginosis (BV), 20 had a C. trachomatis genital infection (CT) and 18 suffered from vulvo-vaginal candidiasis (VVC) due to C. albicans. Clinical and demographic information of the study groups are reported in details in Table 1. No significant differences were found in the age distribution between the different groups of women. Most of the HC and CT-positive women showed a Nugent score 0–3 (76% and 65%, respectively), while 60% of BV subjects were characterised by a Nugent score >7. Women in the VVC group had an intermediate condition (Nugent score 4–6) in 61% of cases. A creamy gray discharge was referred by 85% of BV patients, whereas all subjects suffering from VVC complained from itching and/or whitish discharge. More than half (11/20; 55%) of CT-positive patients were completely asymptomatic. The most common CT serovar in our population was E (11/20; 55%), followed by F (4/20; 20%), G (2/20; 10%), D (2/20; 10%) and K (1/20; 5%).

Table 1

Demographic and clinical characteristics of the women enrolled for the study, subdivided in healthy (HC), bacterial vaginosis (BV), C. trachomatis (CT), and vulvo-vaginal candidiasis (VVC) groups.

	HC (N=21)	BV (N=20)	CT (N=20)	VVC (N=18)	P
Enrolment criteria	No symptoms and negative for microbiological tests	Positive for 3/4 Amsel criteria and Nugent score > 3	Vaginal detection of CT DNA by NAAT	Suggestive symptoms and detection of C. albicans	—
Mean age±SD (years)	26.1±6.3	29.3±7.7	24.3±3.4	27.8±7.2	0.08
Mean BMI±SD (kg/m2)	23.8±2.4	22.9±2.4	23.6±1.8	23.0±2.0	0.38
Amsel criteria
Creamy gray discharge	0/21 (0.0%)	17/20 (85.0%)	0/20 (0.0%)	0/18 (0.0%)	<0.0001
Mean vaginal pH value	4.1±0.3	4.8±0.4	4.2±0.4	4.3±0.5	<0.0001
Positive Whiff testa	5/21 (23.8%)	10/20 (50.0%)	4/20 (20.0%)	3/18 (16.7%)	0.07
Clue cells present	1/21 (4.7%)	19/20 (95.0%)	3/20 (15.0%)	5/18 (27.8%)	<0.0001
Nugent score
0–3	16/21 (76.2%)	0/20 (0.0%)	13/20 (65.0%)	7/18 (38.9%)	<0.0001
4–6	5/21 (23.8%)	8/20 (40.0%)	7/20 (35.0%)	11/18 (61.1%)	0.11
7–10	0/21 (0.0%)	12/20 (60.0%)	0/20 (0.0%)	0/18 (0.0%)	<0.001

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^aThe Whiff test is considered positive when, by adding a drop of 10% KOH to a microscopic slide containing the vaginal secretions, a characteristic ‘fishy’ odour is present.

Vaginal microbiota structure characterization

DNA extracted from vaginal swabs was subjected to next generation sequencing of the V3-V4 hypervariable regions of 16S rRNA gene, which produced an average of 56,273 reads per sample. Alpha-diversity analysis using Shannon index metrics showed significant differences in biodiversity among the four groups (Kruskal-Wallis p-value<0.001, Fig. 1a). In particular, BV samples were characterised by a significant higher microbiota diversity compared to HC (p=0.006) and CT (p=0.006) groups; the comparison between HC and BV resulted significant with the Chao1 index as well (p=0.024). No significant differences were observed with the other metrics used (observed species and Faith’s phylogenetic diversity index).

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Figure 1

Structure of the vaginal microbiota. Microbial composition of vaginal swabs of healthy (HC), C. trachomatis (CT), vulvo-vaginal candidiasis (VVC) and bacterial vaginosis (BV) positive women was analysed. (a) Boxplots of Shannon index (α-diversity). (b) Principal Coordinates Analysis (PCoA) plot based on unweighted Unifrac distance (β-diversity). Each point corresponds to a sample. For each experimental class, the SEM-based confidence ellipse around the centroid is depicted. The first two components of the variance are represented.

Beta-diversity analysis highlighted that each condition was characterized by a peculiar microbiota: principal coordinates analysis (PCoA) showed a significant segregation of the four women groups (adonis p-value<0.001, on both weighted and unweighted Unifrac distances, Fig. 1b). Moreover, a significant separation on unweighted Unifrac distance was evidenced for all pairwise comparisons (all p-values<0.05), being BV the most profoundly different condition compared to the others. Weighted Unifrac distances analysis confirmed that HC and BV possessed a unique microbial composition (all pairwise comparison p-values<0.05), whereas VVC and CT conditions, although different from the other two experimental groups, did not show a significant separation between them (p=0.34). Moreover, within the CT-infected group, no significances were observed, in both alpha and beta diversities, according to Chlamydia serotype (serovar E: 11 patients; non-E, including serovars F, G, D, K: 9 patients).

Taxonomic composition of vaginal bacterial communities

Analyzing taxonomic classification at the phylum level, it was found that all samples were mainly constituted by Firmicutes, Actinobacteria, and Bacteroidetes, although with many significant differences in their abundances across groups (all Kruskal-Wallis p-values<0.001, Fig. 2a). Firmicutes dominated the vaginal microbiota of all women groups, with abundance ranging from 92.4% in HC, to 60.6% in BV-affected women. Besides BV, also VVC-infected women (79.2%), but not CT subjects (89.8%) were significantly depleted in Firmicutes. In contrast, Actinobacteria were significantly more abundant in BV (18.4%), and VVC (12.0%) groups with respect to HC (3.4%) and CT (5.6%). A similar observation was made for Bacteroidetes phylum, which represents 11.5% of abundance in BV women, 5.0% in VVC and only 2.7% and 3.0% in HC and CT subjects, respectively. Fusobacteria phylum was significantly enriched only in BV samples (7.9%), while Proteobacteria mainly characterized the VVC group (2.2%).

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Figure 2

Taxonomic composition of the vaginal microbiota. Stacked bar charts of taxonomy relative abundances at (a) phylum and (b) genus level for healthy (HC), C. trachomatis (CT), vulvo-vaginal candidiasis (VVC) and bacterial vaginosis (BV) positive subjects. Only phyla and genera present at relative abundances >1% in at least 20% (i.e.: ≥16 samples) are reported. Remaining taxa are grouped in the “Other” category.

At a lower taxonomic level, Lactobacillaceae family (Kruskal-Wallis p-value<0.001), almost totally represented by Lactobacillus genus, was found predominant in healthy women (79.2%), as well as in CT (67.5%) and VVC (56.7%) subjects, but profoundly and significantly depleted in BV subjects (18.8%). Lachnospiraceae, Bifidobacteriaceae, and Veillonellaceae families were differently represented across groups (Kruskal-Wallis p-values: p<0.005 for all families, Table 2); in particular, they were significantly more abundant in all the pathological conditions with respect to healthy women.

Table 2

Average relative abundances of main microbial groups (phylum, family, genus).

Level		Group	Relative abundance				P-value
			HC (%)	CT (%)	VVC (%)	BV (%)	Overalla	CT vs. HCb	VVC vs. HCb	BV vs. HCb
Phylum	↓	Firmicutes	92.41	89.78	79.22	60.61	<0.001	—	0.004	<0.001
	↑	Actinobacteria	3.39	5.62	12.02	18.38	<0.001	—	<0.001	<0.001
	↑	Bacteroidetes	2.71	2.99	5.02	11.51	<0.001	—	0.019	<0.001
	↑	Fusobacteria	0.22	0.42	0.54	7.87	0.003	—	—	<0.001
	↑	Proteobacteria	0.57	0.60	2.23	0.41	0.030	—	0.013	—
Family	↓	Lactobacillaceae	79.19	67.50	56.75	18.83	<0.001	—	0.004	<0.001
	↑	Lachnospiraceae	3.16	8.10	6.61	13.67	0.003	<0.001	0.007	0.007
	↑	Bifidobacteriaceae	2.93	4.17	9.56	11.82	<0.001	0.044	<0.001	<0.001
	=	Ruminococcaceae	5.22	6.59	6.29	6.21	—	—	—	—
	↑	Veillonellaceae	0.74	3.72	2.42	10.91	<0.001	0.031	0.004	<0.001
	↑	Prevotellaceae	1.16	1.60	3.76	9.15	<0.001	—	0.002	<0.001
	↑	Coriobacteriaceae	0.32	1.30	2.29	5.96	<0.001	0.031	<0.001	<0.001
	↑	Leptotrichiaceae	0.20	0.41	0.53	7.77	0.002	—	—	<0.001
	=	Uncl. Clostridiales	1.66	1.56	1.44	1.58	—	—	—	—
	↑	Tissierellaceae	1.14	0.29	1.23	3.04	<0.001	—	0.044	<0.001
	↑	Aerococcaceae	0.03	0.24	1.51	2.07	<0.001	0.034	—	<0.001
	↑	Clostridiaceae	0.19	0.69	0.36	2.17	<0.001	0.009	—	<0.001
	=	Bacteroidaceae	0.69	0.91	0.81	0.86	—	—	—	—
	=	Streptococcaceae	0.42	0.50	1.05	0.55	—	—	—	—
	=	Pasteurellaceae	0.03	0.10	1.46	0.02	—	—	—	—
	=	Porphyromonadaceae	0.38	0.13	0.09	0.83	—	—	—	—
	↑	Gemellaceae	0.06	0.06	0.82	0.28	0.012	—	0.013	0.005
	↑	Actinomycetaceae	0.06	0.09	0.10	0.53	0.010	—	0.029	0.006
	↑	Peptostreptococcaceae	0.04	0.01	0.08	0.64	0.011	—	0.017	0.002
Genus	↓	Lactobacillus	79.16	67.45	56.69	18.80	<0.001	—	0.004	<0.001
	↑	Gardnerella	2.72	3.65	7.68	11.44	<0.001	—	0.003	<0.001
	↑	Prevotella	1.16	1.60	3.76	9.15	<0.001	—	0.002	<0.001
	↑	Megasphaera	0.06	2.97	1.04	8.64	<0.001	0.001	0.036	<0.001
	↑	Roseburia	1.09	4.42	3.51	3.51	<0.001	<0.001	<0.001	0.031
	↑	Sneathia	0.18	0.41	0.53	7.76	<0.001	0.020	—	<0.001
	=	Shuttleworthia	0.03	0.54	0.31	7.48	—	—	—	—
	↑	Faecalibacterium	1.49	2.31	2.14	2.09	0.027	0.003	0.041	—
	↑	Atopobium	0.17	1.00	1.94	4.92	<0.001	0.024	0.001	<0.001
	↑	Ruminococcaceae (other)	1.35	1.95	1.86	1.81	0.023	0.003	0.041	—
	=	Uncl. Clostridiales	1.66	1.56	1.44	1.58	—	—	—	—
	=	Uncl. Ruminococcaceae	1.08	1.03	1.02	1.03	—	—	—	—
	↑	Aerococcus	0.03	0.24	1.50	2.06	<0.001	0.021	—	<0.001
	↑	Dialister	0.37	0.56	0.78	2.02	<0.001	0.011	<0.001	<0.001
	↑	Clostridium	0.16	0.65	0.32	2.14	<0.001	0.008	—	<0.001
	↑	Bacteroides	0.69	0.91	0.81	0.86	—	—	—	—
	↑	Blautia	0.35	0.91	0.77	0.74	<0.001	<0.001	0.004	—
	=	Coprococcus	0.61	0.66	0.63	0.59	—	—	—	—
	↑	Streptococcus	0.40	0.49	1.04	0.54	0.045	0.012	0.013	—
	=	Oscillospira	0.65	0.56	0.53	0.58	—	—	—	—
	↑	Ruminococcus	0.54	0.58	0.59	0.56	—	—	—	—
	↑	Bifidobacterium	0.20	0.46	1.28	0.33	<0.001	<0.001	0.005	—
	↑	Uncl. Lachnospiraceae	0.40	0.59	0.54	0.52	0.029	0.006	—	—
	=	Parvimonas	0.20	0.07	0.17	1.39	<0.001	—	—	<0.001
	=	Peptoniphilus	0.35	0.15	0.13	1.09	—	—	—	—
	=	Haemophilus	0.02	0.10	1.42	0.02	—	—	—	—
	↑	Uncl. Coriobacteriaceae	0.10	0.19	0.24	0.89	<0.001	0.023	0.017	<0.001
	=	Akkermansia	0.30	0.39	0.35	0.32	—	—	—	—
	=	Anaerococcus	0.48	0.03	0.46	0.39	—	—	—	—
	↑	Escherichia	0.13	0.33	0.40	0.23	<0.001	<0.001	<0.001	—
	=	Porphyromonas	0.25	0.05	0.01	0.73	—	—	—	—
	↑	Ureaplasma	0.25	0.04	0.41	0.09	<0.001	0.001	—	0.005
Genus	↑	Peptostreptococcus	0.03	<0.01	0.08	0.63	0.018	—	0.024	0.009
	↑	Finegoldia	0.09	0.03	0.45	0.16	0.016	—	0.015	—
	↑	Veillonella	0.08	0.07	0.46	0.08	0.038	—	0.019	—
	↑	Gemella	0.05	0.03	0.32	0.27	<0.001	—	—	<0.001
	↑	Mobiluncus	0.03	0.08	0.02	0.49	0.013	—	—	0.002
	↑	Alloscardovia	<0.01	0.06	0.57	<0.01	0.002	0.041	<0.001	—
	↑	Uncl. Gemellaceae	0.01	0.03	0.49	<0.01	0.019	—	—	0.041
	=	Uncl. Rs-045	<0.01	<0.01	0.04	0.43	—	—	—	—

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For each taxa, significant p-values of the non-parametric Kruskal-Wallis test and of pairwise comparison (Dunn’s test) of women affected by Chlamydia trachomatis infection (CT), vulvo-vaginal candidiasis (VVC) or bacterial vaginosis (BV) versus healthy women (HC) are reported. Arrows indicate the direction of the variation with respect to HC: ↓, decreased; ↑, increased; =, unchanged.

^aP-value of the Kruskal-wallis test for equal medians among groups.

^bP-value of the Dunn’s post-hoc test test for equal medians between each pathological condition (i.e.: CT, VVC or BV) and HC.

Analysing the vaginal microbiota composition at the genus level, the aforementioned progressive depletion in Lactobacillus genus from HC, to CT, VVC, and BV conditions was associated to a corresponding increase in the anaerobe genera Gardnerella, Prevotella, Megasphaera, Roseburia and Atopobium (all Kruskal-Wallis p-values<0.001, Fig. 2b). With respect to HC vaginal microbiota composition, Gardnerella and Prevotella were found significantly increased in BV and VVC women only, whereas Megasphaera, Roseburia and Atopobium were significantly increased in CT subjects too. Moreover, Faecalibacterium was significantly increased in both CT and VVC, but not in BV. Finally, BV vaginal microbiota also showed a significant increase in Sneathia (Table 2). Notably, 4/20 women affected by BV showed the presence of Shuttleworthia at very high relative abundances (range: 29.1–41.5%), compared to an average of 0.23% in other samples. Among other bacteria, Mycoplasma was detected at very low relative abundance (0.07%) and resulted increased (p<0.05) in BV with respect to HC and CT; Chlamydia, on the other hand, was detected only in CT subjects, as expected, although at a very low relative abundance (0.02%) (data not shown).

Species-level analysis

Because of the crucial importance of Lactobacillus species in the vaginal environment, a focus on this genus has been performed (Fig. 3). The Lactobacillus species-level analysis revealed that the vaginal microbiota of healthy subjects was dominated by L. crispatus for up to 61.0% of the total Lactobacillus sequences, while the shift towards a pathological condition was associated to a significant reduction in this species abundance (p=0.011), which represented 41.6% in CT infected women, 33.4% in VVC (p=0.006 vs HC) and 28.5% in BV subjects (p=0.005 vs HC). Such a depletion was reflected in a corresponding increase of L. iners in the shift from the healthy status (20.5%) to CT infection (42.1%), VVC (46.2%, p=0.01 vs HC), and BV (56.7%, p=0.009 vs HC). Interestingly, L. gasseri mainly characterized the VVC group (9.7%, p=0.005 vs HC), whereas BV subjects showed the highest diversity of Lactobacillus species, including L. salivarius, L. helveticus, L. delbrueckii, and L. rhamnosus.

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Figure 3

Species-level characterization of Lactobacillus genus. Sequences classified within Lactobacillus genus were characterized down to species level by classifying them against a custom reference database. Proportions are relative to the total amount of Lactobacillus genus in each group. Only the twelve most-abundant species are represented. Less abundant Lactobacillus species are grouped under “Other Lactobacillus”. Per-sample relative abundances were averaged in each experimental category: (a) healthy (HC), (b) C. trachomatis (CT), (c) vulvo-vaginal candidiasis (VVC) and (d) bacterial vaginosis (BV) positive women.

Besides the Lactobacillus species, the SPINGO-based bioinformatic analysis allowed to further refine the taxonomic classification of several bacterial genera found as differentially abundant in HC vs CT, VVC or BV comparisons. Identified species were Gardnerella vaginalis, Faecalibacterium prausnitzii, Megasphaera elsdenii, Atopobium vaginae, Shuttleworthia satelles and Chlamydia trachomatis. Sneathia genus comprised both S. amnionii and S. sanguinegens. Within Dialister genus, we detected D. succinatiphilus, plus other unidentified members; Prevotella species included, among others, P. bivia, P. amnii, and P. timonensis; the majority (>97%) of Mycoplasma sequences were identified as M. hominis. Finally, no species-level characterization was obtained for Roseburia.

Vaginal metabolites composition for VVC-affected women and metabolite-microbiota correlation

In this paper, we analyzed the metabolites composition of VVC samples by ¹H-NMR, and compared it to the metabolomics data already obtained for the other groups²⁰. Multiple comparison of metabolic profiles revealed a peculiar fingerprint of VVC-affected women, which resulted significantly separated from HC (p<0.02 on both Euclidean and Bray-Curtis distances) and from BV (p<0.002 on both distances). No significant differences were found between VVC and CT groups (Fig. 4).

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Figure 4

Vaginal metabolome analysis. Principal Coordinates Analysis (PCoA) on Bray-Curtis distance showing the separation of the samples according to metabolite profile as obtained from ¹H-NMR analysis of vaginal samples from healthy (HC), C. trachomatis (CT), vulvo-vaginal candidiasis (VVC) and bacterial vaginosis (BV) positive women. The first and second components of the total variance are shown.

When compared to the healthy group, VVC metabolic profile resulted enriched (p<0.05) in TMA-Nox (TMAO), taurine, and methanol, and, similarly to BV samples, in isopropanol, O-acetylcholine and glucose. In addition, significantly lower concentrations (p<0.05) of lactate, 4-hydroxyphenylacetate, phenylalanine, pi-methylhistidine and glycine were detected in VVC and BV women; dimethylamine (DMA) and sarcosine were found significantly diminished in all pathological conditions compared to healthy subjects (Fig. 5).

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Figure 5

Metabolite concentration in vaginal samples. Bar plots showing metabolites concentrations per vaginal samples from healthy (HC), C. trachomatis (CT), vulvo-vaginal candidiasis (VVC) and bacterial vaginosis (BV) positive women. Each bar represents the median metabolite concentration (mmol/L) across replicates per experimental group. Error bars show inter-quartile range (i.e.: 75^th–25^th quantile). Metabolites are grouped according to biochemical type (panels a–f). Stars indicate significant differences vs HC group.

Lactobacillus abundance was strongly positively associated with lactate and 4-hydroxyphenylacetate concentrations (Spearman rank coefficient, ρ: 0.65 for both) and, to a lesser extent, to the concentration of several amino acids, such as isoleucine, leucine, tryptophan, phenylalanine and aspartate (ρ ranging from 0.28 to 0.58), DMA (ρ=0.28), sarcosine (ρ=0.54) and pi-methylhistidine (0.49). On the other hand, Lactobacillus abundance was negatively correlated to formate (ρ=−0.51), acetate (ρ=−0.67), 2-hydroxyisovalerate (ρ=−0.55) and alanine (ρ=−0.43) concentrations (Fig. 6). Other bacterial genera (i.e.: Gardnerella, Prevotella, Megasphaera, Atopobium, Dialister and Clostridium) showed a negative correlation with the aforementioned metabolites positively related to Lactobacillus, with coefficients ranging from −0.24 to −0.64, while they showed a positive correlation to organic acids (i.e.: formate, pyruvate, propionate, acetate, 2-hydroxyisovalerate), amines (i.e.: TMA, putrescine), amino acids (i.e.: proline and alanine) and 5-aminopentanoate (ρ=0.27–0.57). Finally, a third group of bacteria, including Roseburia, Blautia, Streptococcus and Bifidobacterium, was moderately positively correlated to TMAO and taurine (ρ=0.24–0.33). All of these correlation coefficients were statistically significant (p<0.05). Bacterial and metabolite-derived data were characterized by a very high and significant association, as demonstrated by Procrustes analysis (permutation-based p-value of m² goodness-of-fit estimation <0.001) and RV coefficient (0.89, p<0.001, for all phylogenetic levels).

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Figure 6

Correlation between metabolome and microbiota. Heatmap showing the Spearman’s correlation coefficient between metabolites concentration and the relative abundances of the main bacterial genera. Only groups present at >1% of relative abundance in at least one sample were considered. Metabolite and microbial data were clustered using Pearson’s correlation metric and average linkage.

Discussion

In this work, we investigated the modifications that take place in the vaginal microbiome and metabolome during common genital infections, namely VVC and CT infection, in comparison with the thoroughly studied conditions corresponding to health and BV. Data generated by high-performing techniques, i.e. NGS and ¹H-NMR spectroscopy, depicted in details the composition of the vaginal microbiome and metabolome and, notably, their correlations, in order to identify hallmarks of these common genital infections.

To the best of our knowledge, this paper analyzes in depth, for the first time, the vaginal microbiome and metabolome of VVC affected women. VVC represents the most frequent mucocutaneous mycosis caused by yeasts of the genus Candida, with a significant impact on women’s quality of life and associated medical costs, because of the high rate of recurrences and the increasing level of antifungal resistance²¹. In this context, a deep knowledge about the vaginal ecosystem during Candida infections could contribute to better understand VVC pathogenesis and to set up new strategies for its control.

According to the taxonomic and metabolomics analysis, we have found that each of the four conditions herein considered is characterized by a peculiar fingerprint.

Vaginal microbiota of HC was dominated, as expected, by members of the Lactobacillus genus and, in particular, by L. crispatus species. Lactobacillus showed a trend towards a decrease in CT women, associated to a significant increase in Megasphaera, Atopobium, Faecalibacterium and Roseburia. The increase of Faecalibacterium prausnitzii in vaginal flora has already been recognized as a risk factor for C. trachomatis infection acquisition²². Similarly, Balle and colleagues found that some taxa were differentially abundant in the cervico-vaginal microbiota between C. trachomatis-infected and uninfected women, including G. vaginalis, A. vaginae, Dialister spp., Prevotella spp. and Megasphaera (all BV-associated bacteria)²³.

On the other hand, microbiota of BV-affected women was characterized by a major shift in the relative abundance of many groups, such as Lactobacillus (decreased), Gardnerella, Prevotella, Megasphaera, Roseburia, Sneathia, and Atopobium (all increased). Increase of Mobiluncus was also observed, even if at a low relative abundance (0.49% and 0.03% in BV and HC, respectively, p=0.002), confirming evidences from Parolin et al.²⁰. BV condition also presented a peculiar network of co-abundance among bacterial groups, as Lactobacillus was positively correlated to Roseburia CAG, which is, in turn, negatively correlated to Gardnerella group.

VVC microbiota profile was found as significantly altered, too, bearing many features in common with BV: all bacterial genera altered in BV, with the exception of Sneathia and with the addition of Faecalibacterium, showed a significant shift also in VVC, suggesting that Candida spp. infections reflect into a considerable modification of the vaginal ecology. Notably, Faecalibacterium and Roseburia, two major players in the healthy gut ecosystem, are associated in the vaginal environment to a proliferation of opportunistic microorganisms and to a general dysbiosis. It should be remembered that the gut could be the initial source of the vaginal colonisation by Candida spp., with the creation of a persistent intestinal reservoir of the yeast²⁴. Thus, the presence of Faecalibacterium and Roseburia in the vaginal niche could reflect a general translocation of various microorganisms from the gastrointestinal tract to the vagina.

The trend towards the reduction of the relative abundance of L. crispatus and the corresponding increase of L. iners was found as another fingerprint of the ongoing dysbiosis in infectious conditions. L. crispatus has been reported to reduce the adherence of G. vaginalis and C. trachomatis to epithelial cells²⁵, while L. iners has not²⁶. A potential explanation lies in the fact that L. iners does not produce D-lactic acid, as well as H₂O₂, suggesting a lower protective ability against pathogens as compared to other Lactobacillus species²⁷. Indeed, a L. iners-dominated community was associated with an increased risk for the acquisition of C. trachomatis genital infections²². Similarly, an increase of non-H₂O₂-producing L. iners, with a contemporary decrease in H₂O₂-producing Lactobacillus species, has been observed in VVC-positive women²⁸. Thus L. iners is considered to be a transitional species, colonizing after perturbations of the vaginal environment, whereas a L. crispatus-predominant microbiota represents the hallmark of a healthy and stable vaginal status^29,30.

Metabolome analysis by ¹H-NMR spectroscopy provided a further explanation of the ongoing changes in genital infections as compared to HC. The production of lactic acid by Lactobacillus species, L. crispatus in particular (connected to the concentration of its conjugate base lactate), acidifies the pH in the vaginal environment, contributing in keeping the environment in a homeostasis against growth of potentially harming microorganisms³¹. Lactic acid bacteria species are known producers of branched-chain amino acids³², thus the higher concentration of some of them, such as valine, leucine, and isoleucine, is another hallmark of the prevalence of lactobacilli in HC women.

On the other hand, the sharp decrease in lactate concentration in CT, VVC and BV, with the consequent increase of pH, is a marker of dysbiosis: the proliferation of diverse bacterial genera, some of them typical of the gut/fecal microbiota³¹, is further associated to the increased presence of short-chain fatty acids, such as butyrate, propionate and acetate.

Moreover, all three infection conditions are characterized by a significant reduction of DMA concentration, with a corresponding trend towards the increase of TMA (which reaches a significant level in BV), suggesting that vaginal dysbiosis impaired the balance between these two molecules belonging to the same pathway of choline degradation³³.

In accordance with previous investigations^7,20, we found that BV women are characterized by higher levels of biogenic amines and short-chain organic acids. Moreover, we reinforced the hypothesis of the role of Mycoplasma hominis in BV condition³⁴, highlighted by both its increased relative abundance and the positive correlation between Mycoplasma and BV-related metabolites (e.g. formate, putrescine, propionate, acetate).

The vaginal metabolic profiles of CT-infected women show only slight modifications compared to healthy controls, being the reduction in some amino acids and biogenic amines the most significant variations. Presence of C. trachomatis was assessed only in CT subjects, as expected, at a low relative abundance, in accordance with its obligated intracellular localization in the upper genital tract, and not in the vaginal environment.

Other interesting data emerged from the investigation of the vaginal metabolic features in VVC-positive women, in particular the increased concentrations of vaginal glucose. High levels of glucose not only enhance the nutritive substrate of Candida, but also promote the expression of binding molecules in vaginal epithelial cells, thus increasing the adhesion of Candida^35,36.

Notably, the higher levels of glucose found in VVC-positive women could by associated with the reduction of L. crispatus relative abundance in the vaginal environment. Indeed, it has been demonstrated that L. crispatus species shows a high metabolic activity in term of glucose consumption³⁷.

During the switch from a commensal to a vaginal pathogen, Candida spp. produce various extracellular released enzymes, such as proteinases (Saps), phospholipases and hemolysins, that are implicated in adhesion, invasion and destruction of vaginal epithelial cells³⁸. Some of the metabolic variations found in the vaginal ecosystem in VVC-positive women (e.g. higher levels of amino acids) could be ascribed to the tissue damage due to hydrolytic enzymes secreted by Candida.

In conclusion, in this paper, via 16S rRNA-based sequencing and ¹H-NMR spectroscopy, we obtained deeper insights into the vaginal ecosystem of common infections of the female genital tract, such as VVC and C. trachomatis infection, on both a taxonomic and metabolomics point of view. We found that women suffering from VVC and CT conditions were characterized by a vaginal microbiome positioning between eubiosis (healthy women) and dysbiosis (BV-positive subjects), with a depletion of lactobacilli and a corresponding increase in different anaerobe genera (e.g. Gardnerella, Prevotella, Megasphaera, Roseburia, Faecalibacterium and Atopobium). The changes in the bacterial communities occurring during the genital infections correlated to significant alterations in the composition of vaginal metabolome, being the decrease of lactate concentration a common marker of all these infectious conditions.

From a clinical point of view, it has been already shown that several metabolites are significantly associated to BV clinical signs and symptoms, considering that this condition has strong metabolic signatures across multiple metabolic pathways³⁹. Contrariwise, in VVC and CT infections, the quest for specific vaginal signatures (both taxonomic and metabolic) useful for clinical, diagnostic, or prognostic purposes is still an open issue. Our work sheds light on the less explored ecology of VVC and CT conditions, even if further investigations are needed to understand whether the microbial/metabolic alterations we identified precede the infection onset or if the pathogens themselves perturb the vaginal environment.

Methods

Supplementary information

Acknowledgements

Author Contributions

B.V., M.S. and A.M. conceived and supervised the study. C.Ce., C.F., C.P., C.Co., L.L. and T.C. performed the experiments and the statistical analysis. A.D. and V.G. recruited volunteers and collected samples. C.Ce., C.F., C.P., C.Co., B.V., M.S. and A.M. wrote the manuscript. All authors read, reviewed and approved the final manuscript.

Data Availability

Raw reads are available in NCBI Short Read Archive (SRA, http://www.ncbi.nlm.nih.gov/sra) under accession number PRJNA523312.

Competing Interests

The authors declare no competing interests.

Footnotes

References