The human body is home to trillions of microorganisms known as the microbiome, which inhabit sites like the gut, skin, and mouth. The vaginal biome is the specific ecosystem of microorganisms residing in the vagina. Unlike the gut, where high diversity signals health, a healthy vaginal biome is characterized by low bacterial diversity. This environment is not static; it changes in response to numerous factors throughout a person’s life, and understanding its function is important for overall health.
The Healthy Vaginal Microbiome Composition and Protective Roles
A healthy vaginal microbiome is dominated by bacteria from the Lactobacillus genus. These bacteria play a protective role by maintaining an acidic environment. They metabolize glycogen, a sugar released by vaginal epithelial cells under the influence of estrogen, into lactic acid. This process lowers the vaginal pH to a range of 3.8 to 4.5, which is inhospitable to many pathogenic organisms.
The most common species in a balanced state include Lactobacillus crispatus, Lactobacillus gasseri, Lactobacillus iners, and Lactobacillus jensenii. Some Lactobacillus species also produce other antimicrobial compounds, like hydrogen peroxide, to inhibit harmful bacteria. By adhering to the vaginal lining, these beneficial bacteria also create a physical barrier, preventing pathogens from causing infection.
Key Influencers of Vaginal Biome Variation
The vaginal biome’s composition is influenced by a wide array of factors, with hormonal fluctuations being a primary driver. Estrogen levels, which vary with the menstrual cycle, pregnancy, and hormonal contraceptive use, directly impact glycogen availability for Lactobacillus, altering the biome’s stability.
Lifestyle and personal habits also play a role. Sexual activity can introduce new microbes and alter vaginal pH, while hygiene practices like douching can wash away beneficial lactobacilli. Broad-spectrum antibiotics can also reduce Lactobacillus populations, leaving the vagina vulnerable. Other influencing factors include diet, smoking, and psychosocial stress.
Consequences of Vaginal Biome Disruption
When the vaginal biome is disturbed—a state known as dysbiosis—the population of protective Lactobacillus declines and the vaginal pH rises. This allows for the overgrowth of harmful microorganisms and is linked to several health issues. One is bacterial vaginosis (BV), the most common vaginal condition in women of reproductive age, characterized by a “fishy” odor and discharge.
Dysbiosis also increases the risk of vulvovaginal candidiasis, or yeast infections, and can heighten susceptibility to sexually transmitted infections (STIs) like chlamydia, gonorrhea, and HIV. An imbalanced biome is also associated with pelvic inflammatory disease and adverse pregnancy outcomes like preterm birth.
Classifying Vaginal Biomes: Community State Types
To understand variations in vaginal microbial composition, scientists use five main groups called community state types (CSTs). Four of these types are dominated by a single Lactobacillus species: CST I (L. crispatus), CST II (L. gasseri), CST III (L. iners), and CST V (L. jensenii). CST I, dominated by L. crispatus, is considered the most stable and protective state.
The fifth type, CST IV, is distinct because it lacks a significant Lactobacillus population. CST IV features a diverse mix of anaerobic bacteria, is associated with a higher vaginal pH, and is often linked to conditions like bacterial vaginosis.
Vaginal Biome Evolution Throughout Life Stages
The vaginal microbiome changes across different life stages, driven by shifts in hormone levels. Before puberty, low estrogen levels result in a neutral pH and a diverse biome not dominated by lactobacilli. With the onset of puberty, rising estrogen levels foster a Lactobacillus-dominant environment and a lower, more acidic pH.
During pregnancy, the biome often becomes even more stable and dominated by lactobacilli, providing enhanced protection for the developing fetus. After menopause, the sharp decline in estrogen causes the vaginal environment to revert to a state similar to prepuberty, with reduced lactobacilli, a higher pH, and increased microbial diversity.