Lactobacilli, vaginal pH and bacterial vaginosis
Lactobacilli, the resident bacteria that keep the vagina acidic, turn glycogen into lactic acid; losing them lets anaerobic bacteria overgrow, causing bacterial vaginosis (an imbalance of the normal bacteria).
Remember
Lactobacilli turn glycogen into lactic acid to keep the vagina acidic. Douching (washing out the vagina to clean it) can remove them, so less lactic acid is made and anaerobic organisms such as Gardnerella vaginalis can overgrow once the pH is above about 4.5, causing bacterial vaginosis.
Memory aid
lactobacilli are the acid guards of the vagina, and douching sacks the guards
Think of lactobacilli as guards that keep the vagina acidic, and of douching as a way of sacking those guards.
The vagina carries its own community of bacteria, and the main members are the lactobacilli. They are commensals, meaning normal resident microorganisms that are generally harmless. Lactobacilli use glycogen, a stored form of sugar found in the cells lining the vagina, and convert it into lactic acid. This keeps the vaginal pH (the scale of acidity, where lower numbers mean more acid) acidic, typically about 3.8 to 4.5.
Many potential pathogens (disease-causing microorganisms) grow poorly in acid, so this low pH helps to stop them taking hold. It is one example of colonisation resistance, the protection that established resident organisms give against an incoming pathogen.
The balance can be lost. When the vagina is cleaned by douching (washing it out), the lactobacilli can be washed out. Less lactic acid is then made, so the pH rises above about 4.5. Anaerobic bacteria (bacteria that grow without oxygen), such as Gardnerella vaginalis, can then overgrow. That overgrowth is bacterial vaginosis. Douching is a recognised contributing factor, alongside menstruation, semen and not using condoms, which are linked to a more alkaline (less acidic) vaginal pH.
Bacterial vaginosis is a change in the balance of the normal bacteria rather than a conventional infection from outside, and it is not classed as a sexually transmitted infection. It is a common cause of unusual vaginal discharge, typically thin and greyish with a strong fishy smell.
Key facts
- Protective bacteria
- lactobacilli are the main commensals of the vagina.
- Their action
- they convert glycogen into lactic acid.
- Normal vaginal pH
- typically about 3.8 to 4.5, which discourages harmful bacteria.
- Effect of douching
- it can wash away the lactobacilli, so less lactic acid is made.
- After they are lost
- the pH rises above about 4.5 and anaerobic bacteria such as Gardnerella vaginalis can overgrow.
- Discharge
- bacterial vaginosis is a common cause of unusual vaginal discharge, typically thin and greyish with a strong fishy smell.
Exam traps
- Injuring the vaginal lining, or bringing in germs from outside, does not mainly explain how douching predisposes to bacterial vaginosis, because the key problem is the loss of the protective resident bacteria.
- Anaerobic overgrowth in bacterial vaginosis is not caused by extra oxygen: the change that matters is the loss of the lactobacilli, which lets the anaerobes (bacteria that grow without oxygen) increase.
- A healthy vagina is not sterile: it carries its own community of bacteria, and protection comes from the balance of those bacteria, not from removing all organisms.
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