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Microbiology · Other Gram-negative rods

Gardnerella vaginalis !

  • Bacterium
  • Gram-variable
  • For completeness

A bacterium that overgrows when vaginal lactobacilli are lost and causes bacterial vaginosis: gray discharge with a fishy smell.

Updated 2026-11-01

At a glance

Gram
Gram-variable
Shape
Pleomorphic coccobacillus
Oxygen
Facultative anaerobe
Family
Bifidobacteriaceae
Stain
Gram-variable; coats epithelial cells (clue cells)

Morphology

  • Small Gram-variable coccobacillus
  • Forms biofilms on the vaginal epithelium
  • Grows alongside other anaerobes (Prevotella, Mobiluncus, Mycoplasma hominis)

Transmission

  • Not a classic infection: an imbalance of vaginal flora
  • More common with new or multiple sex partners, condomless sex and douching
  • Can be shared between female partners

Reservoir: Human vagina (part of the flora in small numbers)

Isolation precautions

What the care team adds to standard precautions.

  • Standard

Virulence factors

  • Biofilm stuck to epithelial cells: forms clue cells and shields against antibiotics
  • Displaces the lactobacilli that keep the pH acidic
  • Together with anaerobes it makes amines that cause the fishy smell

Diseases it causes

Clinical picture

  • Thin, homogeneous, gray-white discharge with a fishy smell that worsens after sex
  • Little inflammation and hardly any itching
  • Many women have no symptoms
  • In pregnancy: preterm birth, prelabor rupture of membranes and chorioamnionitis

Diagnosis

  • Amsel criteria (3 of 4): homogeneous discharge, vaginal pH above 4.5, positive KOH whiff test and clue cells on wet mount
  • Nugent score on a Gram stain of the discharge
  • Molecular tests when microscopy is not available

Treatment

Nitroimidazoles (oral or vaginal metronidazole; tinidazole or secnidazole) or vaginal clindamycin. Relapses are common. Pregnant women with symptoms are treated; treating a male partner is considered after relapses.

Drug class only; doses and regimens are set by the clinician from the current guideline.

In Pharmacology:

Prevention

  • Avoiding douching
  • Condom use
  • Treating symptoms in pregnancy

Test yourself

Think of the answer, then tap the question to see it.

What are clue cells?

Vaginal epithelial cells coated with coccobacilli such as Gardnerella, blurring their edges.

What are the Amsel criteria for bacterial vaginosis?

Homogeneous discharge, vaginal pH above 4.5, positive KOH whiff test and clue cells; 3 of 4 are needed.

Which treatments are used for bacterial vaginosis?

Nitroimidazoles such as metronidazole (oral or vaginal) or vaginal clindamycin.

Why does treating symptomatic bacterial vaginosis matter in pregnancy?

It is linked to preterm birth, prelabor rupture of membranes and chorioamnionitis.

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← Back to the group: Other Gram-negative rods

Educational content on microorganisms; not a treatment guide and no substitute for a health professional's advice. No doses are given.

Sources: Bacterial Vaginosis (BV) (MSD Manual Professional Edition, 2026); About Bacterial Vaginosis (BV) (CDC, 2023).

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Representative model, for educational purposes only.