# Bacterial Vaginosis and Gardnerella

> Bacterial vaginosis and Gardnerella for NEET-PG Microbiology, covering Amsel criteria, clue cells, Nugent scoring and metronidazole treatment.

- Canonical URL: https://prepelephant.com/topics/neet-pg/microbiology/bacterial-vaginosis-and-gardnerella
- Exam / course: NEET-PG · Subject: Microbiology
- Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team
- First published: 2026-10-14
- Last updated: 2026-10-14
- Reference basis: Ananthanarayan and Paniker's Textbook of Microbiology; Shaw's Textbook of Gynaecology; Mandell, Douglas and Bennett's Principles and Practice of Infectious Diseases
- How to cite: "Bacterial Vaginosis and Gardnerella", PrepElephant, https://prepelephant.com/topics/neet-pg/microbiology/bacterial-vaginosis-and-gardnerella

## Direct answer

Bacterial vaginosis is a polymicrobial imbalance in which protective Lactobacillus species are replaced by an overgrowth of anaerobes, with Gardnerella vaginalis as the most recognised organism involved. It is diagnosed clinically by the Amsel criteria, where three of four features are needed, or by the Nugent score on a Gram-stained vaginal smear. Metronidazole is the standard treatment, and most cases are not sexually transmitted infections in the usual sense.

## What you must remember

- **Gardnerella vaginalis:** a small, Gram-variable coccobacillus that can also be found in some healthy women.
- **Pathogenesis:** loss of Lactobacillus species raises the vaginal pH, allowing overgrowth of Gardnerella, Mobiluncus and other anaerobes.
- **Amsel criteria:** thin homogeneous grey discharge, vaginal pH above 4.5, a positive whiff test and clue cells, with three of four needed for diagnosis.
- **Clue cells:** vaginal epithelial cells covered by coccobacilli, with more than 20 per cent of epithelial cells affected.
- **Nugent score:** a 0 to 10 score on a Gram-stained smear, where 7 to 10 indicates bacterial vaginosis, 4 to 6 is intermediate and 0 to 3 is normal.
- **Symptoms:** thin grey discharge with a fishy odour, especially after intercourse, with little itching or inflammation.
- **Treatment:** oral metronidazole for seven days or metronidazole gel, with clindamycin as an alternative.
- **Pregnancy:** bacterial vaginosis is linked to preterm birth, so symptomatic pregnant women are treated.

## Applying Amsel criteria and the Nugent score

Bacterial vaginosis is often diagnosed at the bedside, and the Amsel criteria can be applied in a few minutes. The steps below move from the history to the laboratory score.

1. **History:** ask about grey discharge and a fishy odour, particularly after intercourse, and note whether itching is absent.
2. **Measure pH:** a vaginal fluid pH above 4.5 is recorded with a pH strip.
3. **Perform the whiff test:** a drop of 10 per cent potassium hydroxide on the discharge releases a fishy amine odour in a positive test.
4. **Examine a saline wet mount:** clue cells in more than 20 per cent of epithelial cells support the diagnosis.
5. **Count the Amsel criteria:** three of the four features present confirm bacterial vaginosis for clinical purposes.
6. **Score the Gram stain:** a Nugent score of 7 to 10 confirms the diagnosis in laboratory and research settings.

The main mimics are trichomoniasis and candidiasis, and a wet mount usually separates them. Trichomoniasis gives a frothy discharge with motile organisms, while candidiasis gives itching with pseudohyphae and no fishy odour.

## Where the exam tries to catch you

Bacterial vaginosis questions turn on the organism, the discharge pattern and the correct drug.

- **Trap:** treating Gardnerella as the single cause of bacterial vaginosis — **Correct:** the condition is a polymicrobial imbalance, and Gardnerella is one of several organisms involved.
- **Trap:** labelling itching with a thick white discharge as bacterial vaginosis — **Correct:** that picture suggests candidiasis, whereas bacterial vaginosis has a thin grey discharge and a positive whiff test.
- **Trap:** choosing fluconazole for bacterial vaginosis — **Correct:** metronidazole or clindamycin is used, and fluconazole is the drug for candidiasis.

## Frequently asked questions

### Why is Gardnerella vaginalis not the sole cause of bacterial vaginosis?
Gardnerella vaginalis is one of several anaerobic and facultative organisms that overgrow when protective Lactobacillus species are lost in bacterial vaginosis. The condition is therefore a polymicrobial imbalance, and Gardnerella vaginalis alone does not explain it.

### What are the Amsel criteria for bacterial vaginosis?
The Amsel criteria are four clinical features: thin homogeneous grey discharge, vaginal pH above 4.5, a positive whiff test and clue cells on microscopy. Bacterial vaginosis is diagnosed when three of these four criteria are present.

### What is the Nugent score in bacterial vaginosis?
The Nugent score is a 0 to 10 score from a Gram-stained vaginal smear, based on counts of Lactobacillus, Gardnerella and Mobiluncus morphotypes. A score of 7 to 10 indicates bacterial vaginosis, 4 to 6 is intermediate and 0 to 3 is normal.

### Which drug treats bacterial vaginosis?
Oral metronidazole for seven days, or metronidazole gel, is the first-line treatment for bacterial vaginosis per current guidance. Clindamycin is an alternative, and routine treatment of the male partner after a first episode is not needed, although newer guidance allows considering partner treatment in recurrent disease.

### Why is bacterial vaginosis important in pregnancy?
Bacterial vaginosis in pregnancy is associated with preterm birth and early pregnancy loss, so symptomatic pregnant women are usually treated. Metronidazole is the usual drug, and decisions in pregnancy follow current obstetric guidance.