Bacterial Vaginosis

Bacterial vaginosis (BV) is the most common cause of abnormal vaginal discharge in reproductive-age women. It is not a classic infection by a single pathogen but a disturbance of the vaginal microbiota in which the normally dominant lactobacilli are replaced by an overgrowth of mixed anaerobic and other bacteria, producing a characteristic discharge and odour with little inflammation.

Definition

Bacterial vaginosis is a polymicrobial clinical syndrome resulting from replacement of the normal Lactobacillus-dominant vaginal microbiota by high concentrations of anaerobic and facultative bacteria, such as Gardnerella vaginalis and associated anaerobes, characterised by abnormal discharge and a raised vaginal pH.

Scope

The entry covers the nature of BV as a microbiome disturbance rather than a single-organism infection, its hallmark microbiological shift, the established clinical and laboratory criteria used to recognise it, and its associations with reproductive and obstetric outcomes. It is a reference description and does not provide diagnostic or treatment instructions.

Core questions

• Why is bacterial vaginosis described as a microbiome disturbance rather than a true infection?
• What microbiological shift defines BV?
• How are the clinical (Amsel) and laboratory (Nugent) criteria used to recognise it?
• What reproductive and obstetric outcomes are associated with BV?

Key concepts

• Loss of Lactobacillus dominance
• Polymicrobial anaerobic overgrowth
• Gardnerella vaginalis and biofilm
• Raised vaginal pH
• Clue cells
• Amsel clinical criteria
• Nugent Gram-stain score
• Absence of marked inflammation (vaginosis vs vaginitis)

Mechanisms

In the healthy vagina, lactobacilli produce lactic acid (and in many women hydrogen peroxide), maintaining a low pH that constrains other organisms. In BV this protective community is depleted and replaced by a dense, often biofilm-associated overgrowth of Gardnerella vaginalis together with anaerobes such as Atopobium, Prevotella, and Mobiluncus species; the result is a rise in vaginal pH, production of amines responsible for the characteristic odour, and the appearance of epithelial cells coated with bacteria (clue cells) (Amsel, 1983; Onderdonk, 2016). Because the process involves comparatively little neutrophilic inflammation, it is termed a vaginosis rather than a vaginitis.

Clinical relevance

BV is clinically relevant because it is a very common cause of vaginal symptoms and because the associated disturbance of the genital microbiota has been linked to increased risk of acquiring sexually transmitted infections, of adverse outcomes after gynecologic procedures, and of obstetric complications such as preterm birth (Onderdonk, 2016; ACOG, 2020). This entry describes those associations for orientation and is not a basis for individual diagnosis or treatment.

Epidemiology

BV is the most frequently identified cause of abnormal vaginal discharge in reproductive-age women, although a substantial proportion of affected women report no symptoms. Prevalence varies by population and is influenced by factors associated with disturbance of the vaginal microbiota (Onderdonk, 2016; ACOG, 2020).

Evidence & guidelines

The clinical composite criteria of Amsel and colleagues and the standardised Gram-stain scoring of Nugent and colleagues are the long-established reference methods for recognising BV (Amsel, 1983; Nugent, 1991). Professional guidance from ACOG and the CDC frames its evaluation and management (ACOG, 2020; Workowski, 2021). This entry summarises these sources for orientation rather than reproducing their recommendations.

History

Recognition of the syndrome evolved from the older term nonspecific vaginitis. The 1983 paper by Amsel and colleagues defined the composite clinical criteria (discharge, raised pH, amine odour, and clue cells) that bear that name, and Nugent and colleagues later standardised a reproducible Gram-stain scoring system, which together became the reference approaches for identifying the condition (Amsel, 1983; Nugent, 1991).

Debates

Is bacterial vaginosis sexually transmitted?

BV is associated with sexual activity and shares some epidemiological features with sexually transmitted infections, but it arises from a disturbance of the resident vaginal microbiota rather than from a single transmissible pathogen, and whether and how it is transmitted between partners remains debated.

Seminal works

• amsel-1983
• nugent-1991
• onderdonk-2016

Frequently asked questions

Is bacterial vaginosis a true infection?

It is better described as a disturbance of the vaginal microbiota than as infection by one pathogen: the protective lactobacilli are replaced by an overgrowth of mixed anaerobic and other bacteria, with relatively little inflammation, which is why it is called a vaginosis rather than a vaginitis.

How is bacterial vaginosis recognised?

Two long-standing reference methods are the Amsel clinical criteria (thin discharge, raised vaginal pH, a fishy amine odour, and clue cells on microscopy) and the Nugent Gram-stain score, which quantifies the shift in bacterial morphotypes.