The internal environment of the vagina, known as the vaginal microbiome, plays a significant role in reproductive health. This environment is naturally acidic, serving as a primary defense mechanism against invading organisms. Maintaining this low acidity relies on the activity of beneficial microbes that keep conditions stable and protective. An imbalance in this microenvironment can disrupt natural defenses, leading to discomfort and increased susceptibility to infection.
The Primary Protector: Lactobacilli
The main organisms responsible for creating the acidic environment are bacteria of the genus Lactobacillus. These beneficial microorganisms are the dominant residents in a healthy vagina, often making up over 70% of the bacterial population. Their presence dictates the health and stability of the entire environment.
Specific species, such as Lactobacillus crispatus, Lactobacillus jensenii, and Lactobacillus gasseri, are particularly effective at maintaining this dominance. These bacteria thrive in the low-oxygen conditions of the vagina and actively suppress the growth of other, potentially harmful microbes. Their metabolic activity forms the foundation of the vagina’s self-regulating defense system.
How Lactic Acid Maintains Vaginal pH
The acidity comes from the continuous production of lactic acid, a byproduct of the Lactobacilli’s metabolism. This process begins with the epithelial cells lining the vaginal wall, which are stimulated by estrogen to store a carbohydrate known as glycogen. When these cells shed, the glycogen is released into the vaginal fluid.
The Lactobacilli then feed on the glycogen, converting it through fermentation into lactic acid. This metabolic waste product lowers the pH level of the vaginal fluid. The resulting healthy pH is maintained between 3.5 and 4.5, which is moderately acidic.
This high acidity acts as a natural barrier, creating an inhospitable environment where most disease-causing bacteria and yeasts struggle to grow and multiply. This microbicidal effect provides a continuous defense against infection.
When the Balance is Lost
A reduction in protective Lactobacilli or decreased lactic acid production causes the vaginal pH to rise above 4.5. This shift away from acidity creates an opening for other microbes to proliferate, leading to a state of imbalance called dysbiosis. When the environment becomes less acidic, a diverse group of anaerobic bacteria, such as Gardnerella vaginalis, can overgrow and replace the beneficial Lactobacilli.
The most common consequence of this change is Bacterial Vaginosis (BV), a condition characterized by a thin, gray or white discharge and a distinct fishy odor. A shift to a less acidic environment also reduces the natural protection against yeast, which can lead to vulvovaginal candidiasis, or a yeast infection. This loss of the protective acidic barrier increases the risk of acquiring sexually transmitted infections as well.
Everyday Factors Affecting Acid Levels
Several daily and physiological factors can disturb the Lactobacilli population, leading to a drop in lactic acid levels and a rise in pH. Douching, the practice of washing out the vagina with water or various solutions, is a significant disruptor because it washes away the beneficial Lactobacilli along with the fluid, raising the pH. Similarly, broad-spectrum antibiotics, while treating infections elsewhere in the body, can unintentionally kill off the protective Lactobacilli in the vagina.
Sexual activity also introduces external factors that interfere with acidity, as semen is naturally alkaline, with a pH of around 8.0, which temporarily raises the vaginal pH. Hormonal fluctuations throughout the menstrual cycle and during menopause also play a role. The decline in estrogen during menopause, for instance, leads to less glycogen being supplied to the vaginal cells, which in turn starves the Lactobacilli and reduces lactic acid production, often resulting in a pH above 5.0.