What Is Bacterial Vaginosis?


A healthy vagina hosts several types of bacteria living in a balanced state. When the balance tips and certain anaerobic (non-oxygen dependent) bacteria predominate, you might notice irritating vaginal symptoms. Bacterial vaginosis can be sexually transmitted but it can also result from factors that upset the vaginal environment. If your doctor diagnoses bacterial vaginosis, he will prescribe antibiotics.


Although often considered a sub-type of vaginitis (inflammation of the vagina), bacterial vaginosis does not cause your body to mount an immune response. Unlike other types of vaginitis like candidal vaginitis (yeast infection) or trichomonal vaginitis (Trichomoniasis), the causative agents of bacterial vaginosis do not actually invade and infect the cells lining your vagina, but merely stick to their surfaces.


Normally, your vagina hosts an abundant population of the aerobic (oxygen dependent) bacteria Lactobacillus acidophilus. L. acidophilus prefers and helps maintain the slightly acidic environment of a normal vagina. If your vaginal pH becomes more alkaline, such as from the presence of semen or menstrual blood, less abundant anaerobic bacteria such as Gardnerella vaginosis or Mycoplasma hominis can proliferate and overrun the L. acidophilus.

Bacterial vaginosis can be sexually transmitted but can also result from poor hygiene, use of harsh cleansers or feminine deodorants or from douching, all of which can upset the normal balance of vaginal bacteria.


Bacterial vaginosis does not trigger an immune response because it is caused by an overgrowth of organisms that are already present in the vagina. That means some women might not notice any symptoms at all. Other women might experience some itching or discomfort.

When you have bacterial vaginosis, your vaginal discharge might appear gray and you might notice a fishy odor. The odor results from the presence of amines, a byproduct of the anaerobic bacteria, and worsens with higher pH, such as after sexual intercourse.


According to the Merck Manual, diagnosis of bacterial vaginosis requires the presence of any three of these four factors: gray vaginal discharge, discharge with pH higher than 4.5, the presence of clue cells or a fishy odor on the “whiff test.”

Your doctor will take several vaginal swabs for tests and examine one under a microscope. A normal vaginal swab contains clear, well-defined cells from the vaginal lining and the abundant large rod cells of L. acidophilus. In bacterial vaginosis, the cells have irregular borders and a dotted appearance, indicating they are covered with small anaerobic bacteria; L. acidophilus rods are scant. These conditions designate the vaginal cells as clue cells.

In the whiff test, your doctor exposes another vaginal swab to high pH to see if it triggers the characteristic fishy odor.


Your doctor will prescribe antibiotics to target the anaerobic bacteria and restore the normal bacterial balance of your vagina. Metronidazole gel or pills or clindamycin cream are the preferred treatments, according to “Primary Care for Women” by Phyllis Leppert and Jeffrey Peipert. The antibiotics start to work within about two days.

About this Author

In 20 years as a biologist, Susan T. McClure has contributed articles to scientific journals such as “Nature Genetics” and “American Journal of Physiology.” She earned her Ph.D. from the University of Maryland. She enjoys educating people about science and the challenge of making complex information accessible.