Bacterial Vaginosis

Bacterial vaginosis (BV) refers to an overgrowth of bacteria in the vagina, specifically anaerobic bacteria. It is not a sexually transmitted infection. It is caused by a loss of the lactobacilli “friendly bacteria” in the vagina. Bacterial vaginosis can increase the risk of women developing sexually transmitted infections.

Lactobacilli are the main component of the healthy vaginal bacterial flora. These bacteria produce lactic acid that keeps the vaginal pH low (under 4.5). The acidic environment prevents other bacteria from overgrowing. When there are reduced numbers of lactobacilli in the vagina, the pH rises. This more alkaline environment enables anaerobic bacteria to multiply.

Examples of anaerobic bacteria associated with bacterial vaginosis are:

  • Gardnerella vaginalis (most common)
  • Mycoplasma hominis
  • Prevotella species

It is worth remembering that bacterial vaginosis can occur alongside other infections, including candidiasis, chlamydia and gonorrhoea.


Risk Factors

There are a number of factors that increase the risk of developing bacterial vaginosis:

  • Multiple sexual partners (although it is not sexually transmitted)
  • Excessive vaginal cleaning (douching, use of cleaning products and vaginal washes)
  • Recent antibiotics
  • Smoking
  • Copper coil

Bacterial vaginosis occurs less frequently in women taking the combined pill or using condoms effectively.

TOM TIP: When taking a history from someone with typical symptoms of bacterial vaginosis, the diagnosis can be quite obvious based on the fishy-smelling discharge. The thing that scores you points in your exams and is critical in practice is to assess for causes and give advice. For example, sensitively ask about the use of soaps to clean the vagina and vaginal douching and provide information about how these can increase the risk. 



The standard presenting feature of bacterial vaginosis is a fishy-smelling watery grey or white vaginal discharge. Half of women with BV are asymptomatic.

Itching, irritation and pain are not typically associated with BV and suggest an alternative cause or co-occurring infection.

speculum examination can be performed to confirm the typical discharge, complete a high vaginal swab and exclude other causes of symptoms. Examination is not always required where the symptoms are typical, and the women is low risk of sexually transmitted infections.



Vaginal pH can be tested using a swab and pH paper. The normal vaginal pH is 3.5 – 4.5. BV occurs with a pH above 4.5.

A standard charcoal vaginal swab can be taken for microscopy. This can be a high vaginal swab taken during a speculum examination or a self-taken low vaginal swab.

Bacterial vaginosis gives “clue cells” on microscopy. Clue cells are epithelial cells from the cervix that have bacteria stuck inside them, usually Gardnerella vaginalis.

TOM TIP: Remember that clue cells on microscopy mean bacterial vaginosis. This is a common association tested in MCQ exams. 



Asymptomatic BV does not usually require treatment. Additionally, it may resolve without treatment.

Metronidazole is the antibiotic of choice for treating bacterial vaginosis. Metronidazole specifically targets anaerobic bacteria. This is given orally, or by vaginal gelClindamycin is an alternative but less optimal antibiotic choice.

Always assess the risk of additional pelvic infections, with swabs for chlamydia and gonorrhoea where appropriate.

Provide advice and information about measures that can reduce the risk of further episodes of bacterial vaginosis, such as avoiding vaginal irrigation or cleaning with soaps that may disrupt the natural flora.

TOM TIP: Whenever prescribing metronidazole advise patients to avoid alcohol for the duration of treatment. This is a crucial association you should remember, and something examiners will look out for when you are explaining the treatment to a patient. Alcohol and metronidazole can cause a “disulfiram-like reaction”, with nausea and vomiting, flushing and sometimes severe symptoms of shock and angioedema.



Bacterial vaginosis can increase the risk of catching sexually transmitted infections, including chlamydia, gonorrhoea and HIV.

It is also associated with several complications in pregnant women:

  • Miscarriage
  • Preterm delivery
  • Premature rupture of membranes
  • Chorioamnionitis
  • Low birth weight
  • Postpartum endometritis


Last updated July 2020
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