Vaginal Health: pH and Microbial Flora
1. Definition and Overview
"Vaginal flora," "vaginal microbiota," and "vaginal microbiome" all refer to the microorganisms that colonize the vagina. They were first described by the German gynecologist Albert Döderlein in 1892 and are part of the broader human flora. The amount and type of bacteria present have significant implications for an individual's overall health.
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, Lactobacillus dominance appears to be unique to humans; while the relative abundance of lactobacilli in the human vagina is typically greater than 70%, in other mammals lactobacilli rarely comprise more than 1% of vaginal microbiota.
Normal vaginal pH is considered to be under 4.5, with a range of 3.8 to 4.4. High estrogen states, as seen during puberty and pregnancy, promote the preservation of a homeostatic (eubiotic) vaginal microenvironment by stimulating the maturation and proliferation of vaginal epithelial cells and the accumulation of glycogen. A glycogen-rich vaginal milieu is a haven for the proliferation of Lactobacilli facilitated by the production of lactic acid and decreased pH. Lactobacilli and their antimicrobial and anti-inflammatory products, along with components of the epithelial mucosal barrier, provide an effective first-line defense against invading pathogens including bacterial vaginosis, aerobic vaginitis-associated bacteria, viruses, fungi, and protozoa.
2. The Microbial Ecosystem: Composition and Function
2.1 Key Species
The primary colonizing bacteria of a healthy individual are of the genus Lactobacillus (90–95%), the most common being L. crispatus, L. iners, L. jensenii, and L. gasseri. These microorganisms act as probiotics and inhibit the overgrowth of other bacterial species through several direct and indirect antipathogenic mechanisms — directly by producing active components such as lactic acid and hydrogen peroxide (H₂O₂), which kill or inhibit pathogens; and indirectly by forming microcolonies that adhere to epithelial cells and create a physical barrier against the adhesion of certain microorganisms, while also promoting the stimulation of host defense mechanisms against infections and sexually transmitted diseases.
At puberty, rising levels of estrogen promote the maturation, proliferation, and accumulation of glycogen in vaginal epithelial cells. Glycogen is catabolized by human α-amylase to maltose, maltotriose, and α-dextrines, which are then metabolized to lactic acid by Lactobacillus species. This creates an acidic environment (pH 3.5–4.5) conducive for the growth of Lactobacilli at the expense of other anaerobic bacterial species.
Not all Lactobacillus species confer equal protection. While L. crispatus is consistently linked to low genital inflammation and a reduced risk of adverse outcomes, L. iners is frequently associated with an unstable microbiome. Increasing evidence suggests that L. iners is a transitional species that colonizes after the vaginal environment is disturbed and offers overall less protection against vaginal dysbiosis, and subsequently leads to BV, sexually transmitted infections, and adverse pregnancy outcomes. Accordingly, under certain conditions, L. iners is a genuine vaginal symbiont, but it also seems to be an opportunistic pathogen.
2.2 Community State Types (CSTs)
The vaginal microbiota can be classified into five major community state types (CSTs) based on bacterial content. CST I is dominated by L. crispatus, CST II by L. gasseri, CST III by L. iners, and CST V by L. jensenii. CST IV is exceptional, because it is not dominated by any Lactobacillus species; instead, it consists of different anaerobic species, such as Gardnerella vaginalis and Prevotella spp. CST I is common in healthy individuals, whereas CST III and IV are associated with dysbiosis and infection; CST III-B, IV-A, IV-B, and IV-C0 are prevalent in patients with bacterial vaginosis (BV).
2.3 Additional Microbial Inhabitants
Other bacterial species are frequently found in the vagina, such as gram-positive cocci including Atopobium vaginae, Peptostreptococcus spp., Staphylococcus spp., Streptococcus spp., and Bacteroides spp., Fusobacterium spp., Gardnerella vaginalis, Mobiluncus, Prevotella spp., and gram-negative enteric organisms such as Escherichia coli. Mycoplasma and Ureaplasma are frequently found in the vagina. Some obligate and facultative anaerobic bacteria are associated with BV.
The microbial composition of the human vagina differs from that of all other mammals, likely as a consequence of the unique composition of vaginal constituents that promote the selective maintenance and proliferation of distinct bacterial species. Similarly, variations between individual healthy women in genetic, environmental, and medical variables also modify the vaginal lumen composition.
3. Body Systems Involved
Vaginal pH and flora are not governed by the reproductive tract in isolation; they are shaped by the interaction of multiple physiological systems:
- Endocrine system: Particularly, Lactobacillus abundance changes by level of glycogen, which deposits in epithelial cells upon estrogen stimulation. Both premenarchal girls and postmenopausal women are less likely to have communities dominated by Lactobacillus; one commonality between these two age groups is their propensity to have lower levels of circulating estrogen than reproductive-age women.
- Immune system: Lactobacilli and their antimicrobial and anti-inflammatory products along with components of the epithelial mucosal barrier provide an effective first-line defense against invading pathogens. Vitamin D promotes the expression of antimicrobial peptides by activating the vitamin D receptor (VDR), improves epithelial barrier function, and inhibits pathogen colonization.
- Acid-base transport systems: In the vagina, pronounced compartmentalization in terms of pH may depend not only on the vaginal microbiota but also on acid–base transporters, such as Na⁺/HCO₃⁻-cotransporters, Na⁺/H⁺-exchangers, and epithelial proton pumps, which are abundant throughout the vaginal epithelium.
- Gut–vaginal axis: Fluctuations in gut microbiota from dietary carbohydrate intake may alter the vaginal microbiota.
4. Dysbiosis: Definition and Clinical Presentations
Reduced Lactobacillus levels can lead to dysbiosis, associated with complications like bacterial vaginosis, preterm birth, and higher risks of STIs.
Bacterial vaginosis (BV), the most common vaginal infection among women of childbearing age, is characterized by the disruption of vaginal flora consisting of dominant physiologic Lactobacillus species to pathologic anaerobic and facultative bacterial species, such as Gardnerella vaginalis, Prevotella bivia, and Atopobium vaginae. The estimated prevalence of BV in the general population is between 23 and 26% worldwide and reaches up to 33% and 31% in Black and Hispanic women, respectively.
Bacterial vaginosis itself is a risk factor for pelvic inflammatory disease, HIV, other STIs, and other obstetric disorders. Disturbance of the vaginal niche with a non-lactobacillary microbiota is associated with susceptibility to some diseases, such as obstetric alterations and infertility, resulting in failure in natural pregnancies and increased demand for assisted reproduction treatments.
Vulvovaginal atrophy (VVA) involves the thinning of the vaginal epithelium, less exfoliation of cells into the vagina, and a rise of vaginal pH between 5 and 7.5, ultimately resulting in a loss of lactobacilli and an overgrowth of pathogenic bacteria such as staphylococci.
5. Contributing and Associated Factors
5.1 Age and Hormonal Life Stage
During childhood, the vaginal microbiome is highly diverse, dominated by anaerobes including Bacteroides, Prevotella, Actinomyces, and Fusobacterium, as well as some aerobic bacteria like Staphylococcus aureus and Staphylococcus epidermidis. The absence of estrogen production during the prepubertal phase results in an alkaline vaginal pH and decreased protective lactobacilli concentration, creating an environment conducive to the overgrowth of fecal or oropharyngeal bacteria.
The overall vaginal microbiome composition is strongly tied to age, childbirth, and menstrual cycle phase. There are significant associations between menopause stage and CST, and between vulvovaginal atrophy and CST. Perimenopausal women are more likely to be classified as CST IV-A or the L. gasseri CST, whereas postmenopausal women are often classified as CST IV-A. CSTs dominated by L. crispatus and L. iners are more prevalent in premenopausal women.
5.2 Race and Ethnicity
Vaginal bacterial communities dominated by Lactobacillus spp. were found in 80.2% and 89.7% of Asian and White women, respectively, but just 59.6% and 61.9% of Black and Hispanic women, respectively. The occurrence of communities with low proportions or no detectable Lactobacillus species was elevated in Hispanic (38.1%) and Black (40.4%) women compared to Asian (19.8%) and White (10.3%) women. Vaginal pH was also found to differ among ethnic groups, with the overall median vaginal pH of Black and Hispanic women being slightly elevated over what is considered normal. The biological, social, and environmental determinants of these differences remain an active area of investigation.
5.3 Lifestyle and Behavioral Factors
Lifestyle factors including diet, sexual behavior, hygienic practice, contraceptives, smoking, stress, and obesity all affect vaginal microbiome composition.
- Douching: Douching during menstruation significantly increased the risk of dysbiosis with an odds ratio of 2.6 (95% CI 1.0–6.5), compared to patients without menstruation. Moreover, vaginal douching appeared to promote Candida albicans infections, probably due to a proinflammatory vaginal immune environment.
- Smoking: Cigarette smoking has been associated with both the diagnosis of bacterial vaginosis and a vaginal microbiota lacking protective Lactobacillus spp.
- Alcohol: Alcohol intake was positively associated with CST IV and specifically correlated with higher levels of Gardnerella and Ureaplasma.
- Sexual behavior: Oral contraception and a stable male sexual partner were found to favor Lactobacillus colonization, acting as a protective factor. Conversely, non-hormonal contraception and unprotected or non-penile/vaginal sexual activity increased the incidence of vaginal inflammation and bacterial vaginosis by disturbing the vaginal microbiota and reducing Lactobacillus abundance.
- Psychosocial stress: Sexual behaviors, smoking, alcohol consumption, and psychosocial stress were associated with an increased susceptibility to bacterial vaginosis, STIs, and severe pelvic inflammatory diseases due to a modified vaginal microbiota.
- Antibiotic use: Some possible risk factors for bacterial vaginosis include vaginal douching, multiple sexual partners, recent antibiotic use, cigarette smoking, and the use of an intrauterine device.
5.4 Contraceptive Choices
Both hormonal and non-hormonal contraceptives may play a role in BV occurrence. A large-scale cross-sectional study (N = 16,314) found that BV is more common in women using the non-hormonal copper intrauterine device (14.8%) than women with hormonal IUDs (9.7%) and women who are not IUD users (11.1%). The module containing Lactobacillus crispatus, Lactobacillus jensenii, and Limosilactobacillus taxa was positively linked to oestrogen levels and contraceptive use.
6. Nutrients, Herbs, and Natural Ingredients
6.1 Probiotic Lactobacillus Strains
Traditional and Historical Background
The normal physiological vaginal microbiota was initially described in 1892 by Albert Döderlein as homogenous, consisting of only gram-positive bacilli (Döderlein's bacilli), believed to originate from the gut and currently known to be a part of the genus Lactobacillus. The consumption of fermented dairy products such as yogurt as folk remedies for vaginal infections has a long informal history across multiple cultures, predating modern microbiological understanding.
Scientific Evidence
A 2024 systematic review that included randomized controlled trials (RCTs) from 2014–2024 on probiotic treatment for bacterial vaginosis incorporated 16 RCTs. The findings revealed that Lactobacillus rhamnosus TOM 22.8 (10×10⁹ CFU/day for 10 days) was the most effective strain and dose, significantly improving Nugent scores, vaginal pH, and microbiota composition and reducing BV recurrence rate.
Lactobacillus rhamnosus GR-1 and Lactobacillus reuteri RC-14 are well-characterized probiotic strains used in combination to prevent and treat BV. The use of probiotic capsules containing Lacticaseibacillus rhamnosus GR-1 and Limosilactobacillus reuteri RC-14, alongside antibiotics, has been linked to improved BV cure rates.
However, results are not uniformly positive. A parallel controlled trial of 126 Chinese women with BV assigned them to metronidazole alone or to the adjunctive probiotic (GR-1 and RC-14 orally for 30 days). There was no significant difference in the 30-day total cure rate between the adjunctive probiotic group (57.69%) and the metronidazole group (59.57%). This mixed evidence underscores the strain-specific, population-specific, and dose-specific nature of probiotic effects on the vaginal microbiome.
In a landmark study, vaginally-administered probiotic L. crispatus CTV-05 use post-metronidazole treatment led to a 34% decreased risk of recurrent BV over 12 weeks. This finding highlights the potential superiority of vaginal-route administration and species matched to native vaginal flora.
Evidence strength: Moderate. Multiple RCTs support benefit, but results are heterogeneous by strain, route, population, and adjunct treatment. Systematic reviews call for larger, more standardized trials.
6.2 Vitamin D
Traditional and Historical Background
Vitamin D's role in vaginal health is a modern scientific insight rather than a traditional herbal or nutritional claim. Its relevance emerged from epidemiological observations in the early 21st century, particularly noting higher BV rates in populations with greater vitamin D deficiency.
Scientific Evidence
Vitamin D, acquired from both dietary sources and UV-B radiation, may play a role in BV occurrence. Researchers have examined its effects with conflicting results. Multiple studies among pregnant women have found that vitamin D deficiency correlates with increased BV risk; however, a large-scale longitudinal study (N = 2,337) did not find a significant relationship between vitamin D deficiency and BV among non-pregnant women. Additionally, a randomized trial of vitamin D supplementation that demonstrated a significant increase in serum vitamin D in treated participants did not demonstrate a significant reduction in BV incidence vs. placebo (65% vs. 48%).
One placebo-controlled randomized clinical trial found a cure rate of asymptomatic BV of 63.5% in the vitamin D intervention group compared to 19.2% in the control group (P <0.001).
Evidence strength: Preliminary and mixed. Observational data among pregnant women suggest an association between low vitamin D and BV risk. Single interventional studies show promise, but the largest longitudinal studies in non-pregnant women do not confirm this association. More high-quality RCTs are needed.
6.3 Dietary Fiber
Traditional and Historical Background
Dietary fiber has long been promoted in various traditional and naturopathic frameworks as a promoter of gut health. Its extension to vaginal health is a product of contemporary gut–vaginal microbiome research rather than historical tradition.
Scientific Evidence
Two recent studies demonstrated that diets rich in fiber are significantly correlated with decreased risk of BV, with odds ratios of 0.22 and 0.49. Higher intake of total carbohydrates, vegetable proteins, fiber, and starch was negatively correlated with Gardnerella spp., suggesting these nutrients may help protect against vaginal dysbiosis.
One study surveying 104 reproductive-aged women found that diets richer in fiber were associated with decreased BV risk; however, researchers could not isolate the source of fiber driving this association, as results were null when examining fiber from beans, grains, and fruits/vegetables separately.
Evidence strength: Preliminary. Cross-sectional and observational data show consistent directional associations, but causality is not established, and the specific type(s) of fiber most relevant remain unclear.
6.4 Betaine
Scientific Evidence
Deficiencies in the micronutrient betaine, found in seafood and spinach, may also correlate with incident BV.
Evidence strength: Very preliminary. Limited observational data. No clinical trials identified.
6.5 Omega-3 Fatty Acids
Scientific Evidence
A greater intake of α-linolenic acid (an omega-3 fatty acid from plant sources such as nuts and seeds) was negatively associated with CST III, suggesting a healthier L. crispatus-dominated environment. This finding emerged from a study of 113 healthy young women with a median age of 21 years, and represents an early-stage dietary association.
Evidence strength: Preliminary, based on observational correlational data. No RCTs specific to omega-3 supplementation and vaginal flora composition have been identified in the peer-reviewed literature searched.
6.6 Vitamins A, C, and E
Scientific Evidence
It has been shown that deficiencies in vitamins A, C, E, and D may increase susceptibility to BV due to their roles in immune function, antioxidative defense, and vaginal epithelial health. Increased fruit and vegetable consumption, which provides these essential vitamins alongside phytochemicals such as betaine, correlates with a reduced risk of BV.
Evidence strength: Preliminary. Evidence derives mainly from observational and epidemiological studies; intervention studies specifically targeting these vitamins for vaginal health are limited.
6.7 Boric Acid
Traditional Use
Boric acid has been used as an intravaginal antiseptic agent since the early 20th century, particularly for recurrent vulvovaginal candidiasis (VVC) and BV. It is not an herbal or nutritional remedy in the conventional sense but has a well-documented history as a topically applied natural compound.
Scientific Evidence
Boric acid was compared with multiple antifungal agents; as monotherapy, boric acid was studied in seven studies. The mycologic cure rates varied from 40% to 100% in patients treated with boric acid; four of the nine included case series reported statistically significant outcomes regarding cure rates. None of the included studies reported statistically significant differences in recurrence rates. Regarding adverse effects, vaginal burning sensation (in fewer than 10% of cases), water discharge during treatment, and vaginal erythema were identified in seven studies.
Boric acid has been suggested as a safe, alternative, economic option for women with recurrent and chronic symptoms of vaginitis when conventional treatment fails because of the involvement of non-albicans Candida spp.
Evidence strength: Moderate for recurrent VVC caused by non-albicans or azole-resistant strains. Case series and comparative studies support use. Clinical evidence for BV is more limited. Oral ingestion of boric acid is toxic; it is studied only as an intravaginal preparation.
6.8 Garlic (Allium sativum)
Traditional Use
Garlic has been used across multiple traditional medicine systems — including Ayurveda, traditional European herbalism, and folk medicine in South and East Asia — as an antimicrobial agent. Vaginal insertion of raw garlic cloves and consumption of garlic extracts for vaginal infections represent longstanding self-treatment practices.
Scientific Evidence
Garlic has strong antibacterial properties and has long been used as a home remedy for bacterial vaginosis. A 2014 study compared the use of garlic tablets and oral metronidazole in treating the condition, with results showing that taking a garlic supplement tablet could be an option for treating bacterial vaginosis. However, this represents a single small study with significant methodological limitations.
Garlic has demonstrated antimicrobial efficacy in various studies, but direct high-quality clinical trial evidence for its effect specifically on vaginal pH and microbiome composition is lacking. In vitro and animal research cannot be directly extrapolated to vaginal use in humans.
Evidence strength: Very preliminary. One small clinical comparison exists; robust RCT evidence for vaginal or oral garlic preparations specifically targeting vaginal flora is absent. Traditional use is culturally widespread but not validated by the current clinical evidence base.
6.9 Tea Tree Oil (Melaleuca alternifolia)
Traditional Use
Tea tree oil originates from the Australian native plant Melaleuca alternifolia and has been used in Australian Aboriginal traditional medicine and subsequently in Western naturopathy as a topical antimicrobial agent. Its use for vaginal infections has circulated in folk and naturopathic contexts for decades.
Scientific Evidence
According to older research, limited anecdotal claims suggest that tea tree oil may help treat bacterial vaginosis. However, there is no scientific evidence that tea tree oil or products containing it can help with the condition. Tea tree oil treatments may cause allergic reactions in some people and are not safe for use during pregnancy.
Essential oils like tea tree oil have been explored for their antifungal and antibacterial properties, but scientific validation is limited.
Evidence strength: Very weak for vaginal applications. In vitro and animal studies show antimicrobial effects against Candida; human clinical trial evidence for vaginal use is absent. Topical application carries documented risks of allergic contact dermatitis.
7. Dietary Factors and the Vaginal Microbiome
7.1 Overall Dietary Pattern
A healthy vaginal environment is typically dominated by Lactobacillus species, which maintain a low pH that inhibits pathogenic bacterial growth. Emerging evidence suggests that diet may significantly influence the composition and function of the vaginal microbiota, offering a modifiable risk factor for BV.
In recent years, researchers have begun to theorize that nutrition is another recognized factor for BV. While little is known about how nutrition may impact vaginal homeostasis, gut microbiome studies have revealed the surprising effect of diet on the composition and function of bacterial communities, which appears to have a deep impact on human well-being and related diseases.
7.2 Glycemic Load and Sugar Intake
It was found that the development of abnormal vaginal flora was promoted by dietary intake of simple sugars, and that maintaining a low dietary glycaemic load would benefit the vaginal area by preventing pathogenic microorganisms from becoming established and reducing the inflammatory response.
High glycemic diets appear to promote BV through increased oxidative stress.
7.3 Fermented Foods and Yogurt
In a cohort of pregnant Caucasian women, a lactobacilli-dominated vaginal microbiota was negatively correlated with a higher pre-pregnancy intake of animal-sourced proteins, whereas a higher pre-pregnancy consumption of total carbohydrates and sugars seemed to be a protective factor for vaginal health. Nevertheless, exhaustive data on the impact of dietary macronutrient intake on the composition of the vaginal environment are still lacking, and many aspects remain to be fully elucidated.
7.4 Animal Protein and Dietary Composition
L. crispatus was linked to specific protective compounds, including branched-chain amino acids (leucine, isoleucine) and the antioxidant 4-hydroxyphenyllactate, suggesting potential roles beyond just lactic acid production.
7.5 Alcohol
Studies have linked higher alcohol intake with an increased risk of bacterial vaginosis and a shift away from a Lactobacillus-dominant vaginal microbiota.
8. Lifestyle Factors Discussed in Authoritative Sources
Risk factors for BV include some sociodemographic factors, including race, and lifestyle/behavioral factors such as smoking, contraceptive use, douching, sexual behavior, and stress.
- Douching: Douching has been noted as a risk factor for subsequent development of pelvic inflammatory disease and STIs. A study of 72 individuals suggested that those with BV who douche were more likely to have a proinflammatory immune signature than those with BV who did not douche. Likewise, BV-associated douching was associated with higher cytokine levels compared with those without BV regardless of douching behavior. Together, these studies present circumstantial evidence that douching should be discouraged.
- Hormonal contraception: Hormonal contraception was found to promote eubiosis of the vaginal microbiota.
- Psychosocial stress: The vaginal microbiome is a dynamic ecosystem important for women's health. Its composition has been associated with risk for menopausal symptoms, sexually transmitted infections, gynecologic cancer, and preterm birth. Conventional risk factors for a vaginal microbiome linked with these adverse health outcomes include sexual behaviors, hygiene practices, individual social factors, and stress levels.
- Clothing and hygiene: Various factors may contribute to bacterial vaginosis development, including frequent bathing, douching, smoking, multiple sexual partners, use of over-the-counter intravaginal hygiene products, high stress levels, and increased frequency of sexual activity.
- Nutritional status and obesity: A growing body of evidence supports a correlation between low nutritional status, as well as obesity, with higher risks of BV and coincident infections.
9. Pregnancy-Specific Considerations
Pregnancy alters the microbiota with a reduction in species/genus diversity. Pregnant women may be more susceptible to BV than the general population, particularly during early pregnancy. Aside from causing urogenital infections and pelvic inflammatory diseases, having BV during pregnancy may lead to numerous adverse obstetric outcomes, such as preterm birth, late miscarriage, and intrauterine fetal death.
Trials have explored Lactobacillus-based probiotic supplementation for clearance of high-risk human papillomavirus and enhancement of fertility outcomes, indicating that probiotic treatment can have benefits that extend beyond BV clearance.
10. Limits of Current Evidence
The direction and magnitude of immune reactions to microorganisms present in the vagina, responses to stress and non-infectious stimuli, coupled with medical and pregnancy history and environmental exposures, can greatly differ between women. Adaptations to both internal and external pressures will determine the ability of select resident vaginal bacteria to numerically dominate, and therefore the definition of a "normal" vaginal microbiota will substantially differ between individual healthy women.
The microbial composition of the vagina in some women is highly dynamic due to several predisposing host factors that eventually affect host–microbial interaction. To date, the single root cause for vaginal dysbiosis — should there be one — remains to be identified.
Daily habits and lifestyle can influence the composition of the vaginal microbiota, thereby affecting vaginal health. Disturbances in the vaginal microbiota could be associated factors for STIs and vaginosis. However, much of the evidence is from observational studies in specific populations, making broad dietary or supplement recommendations premature.
References
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