Gardnerella vaginalis: Bacterial Vaginosis, Clue Cells, and Lab Diagnosis

A woman comes to the clinic with a thin, greyish-white vaginal discharge and a fishy smell that she notices most after sex. What is striking is what she does not have: no itching, no soreness, no redness. This is not the intense inflammation of a yeast infection. That combination, a smelly discharge with almost no inflammation, is the clue to bacterial vaginosis, and it points to a shift in the vaginal bacteria rather than a classic infection. The organism most associated with that shift is Gardnerella vaginalis, and understanding why it causes discharge and odor but little inflammation is the key to this whole topic.

Gardnerella vaginalis is the organism most closely linked to bacterial vaginosis (BV), the commonest cause of abnormal vaginal discharge in women of reproductive age. But Gardnerella is an unusual "pathogen," and getting that straight is the single most useful idea on this page: BV is not one organism invading and infecting. It is a change in the whole community of bacteria that normally live in the vagina.

What bacterial vaginosis really is: a shift in the flora

The healthy vagina is dominated by Lactobacillus species. These bacteria produce lactic acid, which keeps the vaginal pH low (below about 4.5), and this acidity holds other organisms in check. The low pH is the vagina's own defense.

Bacterial vaginosis is what happens when that defense is lost. The lactobacilli decline, the pH rises, and a mixed population of other bacteria overgrows in their place, including Gardnerella vaginalis along with strict anaerobes such as Prevotella, Mobiluncus, and Atopobium. This is why BV is called a dysbiosis (a disturbance of the normal flora) rather than a straightforward infection.

Two consequences follow directly, and they explain the whole clinical picture:

• The fishy odor. The overgrowing anaerobes produce amines (putrescine, cadaverine, trimethylamine). These amines smell fishy, and they become more volatile, and therefore more noticeable, when the pH rises further, which is why the smell is worse after semen (alkaline) or after adding alkali in the laboratory (the whiff test below).
• Little inflammation. Because BV is an overgrowth of the flora rather than an invasive infection, it provokes little of the immune response that causes redness, swelling, and pain. This is why "vaginosis" (a condition of the flora) is the right word, not "vaginitis" (inflammation of the vagina). The relative absence of inflammation and of pus cells is itself a diagnostic clue.

(The full biology of the healthy Lactobacillus-dominated ecosystem, and what disrupts it, will be covered in a dedicated normal vaginal flora article; this page keeps the mechanism only as far as it is needed to understand BV.)

Is Gardnerella the cause of BV, or a marker?

This is where students, and even textbooks, get tangled. Gardnerella vaginalis can be found in many healthy women without BV, and BV involves many organisms, not Gardnerella alone. So Gardnerella is best understood as the marker organism of BV, the one most consistently and heavily present in the shifted flora, and the one whose adherence to epithelial cells produces the diagnostic clue cells, rather than as a lone pathogen that "causes" the condition the way Shigella causes dysentery. Modern work suggests Gardnerella has a real initiating role (it adheres to the vaginal lining and helps build the biofilm that other anaerobes join), but the disease itself is the community shift, not the single organism.

What Gardnerella vaginalis looks like

• Gram stain: a pleomorphic rod or coccobacillus that is Gram-variable (it has a thin Gram-positive-type cell wall but often stains Gram-negative or variable). The Gram-variable behavior is a distinctive and testable feature.
• Non-motile, non-spore-forming, non-capsulate
• Fastidious: it needs enriched media and grows slowly
• Catalase-negative and oxidase-negative
• Culture is not usually done for diagnosis (explained below), because Gardnerella can be present without disease, so growing it does not by itself confirm BV.

Is bacterial vaginosis a sexually transmitted infection?

BV is strongly associated with sexual activity: it is more common in sexually active women, and a new or multiple partners raise the risk. But it is not a classically transmitted infection in the way gonorrhea or chlamydia are.

It can occur in women who are not sexually active, it is not "caught" from a partner as a single pathogen, and treating male partners does not prevent recurrence. It is best described as sexually associated rather than sexually transmitted.

It is often discussed alongside the sexually transmitted infections because it presents as vaginal discharge and is part of the same clinical workup; for that wider context and how BV sits among the causes of vaginal discharge, see the overview of sexually transmitted infections.

Clue cells: the signature of BV

The clue cell is the microscopic hallmark of bacterial vaginosis and the finding this organism is best known for.

A clue cell is a vaginal epithelial (squamous) cell so heavily coated with adherent Gardnerella and other bacteria that its edges look fuzzy, granular, and obscured. Instead of a clean, sharp cell border, the whole surface is stippled with bacteria, and the border disappears under them. On a saline wet mount or a Gram-stained smear, clue cells stand out against the background.

Two points make clue cells diagnostic rather than incidental:

• The bacteria are adherent to the cell, densely covering it, not just floating nearby. That adherence reflects Gardnerella's attachment to the vaginal lining.
• In BV, clue cells appear against a background of few or no pus cells, consistent with the minimal inflammation of a dysbiosis. A smear full of pus cells points away from BV toward an inflammatory cause.

Laboratory diagnosis

BV is diagnosed by a combination of clinical and microscopic findings, not by culturing Gardnerella (which can be present without disease). Two established systems are used: the Amsel criteria at the bedside and the Nugent score in the laboratory.

The Amsel criteria (clinical)

BV is diagnosed when at least three of the following four are present:

1. Thin, homogeneous, greyish-white discharge coating the vaginal walls.
2. Vaginal pH above 4.5 (tested with pH paper). This reflects the loss of the acid-producing lactobacilli and is one of the most useful single findings.
3. Positive whiff (amine) test: adding a drop of potassium hydroxide (KOH) to the discharge releases a fishy amine odor. The alkali volatilizes the amines made by the anaerobes. For the KOH method itself, see KOH preparation.
4. Clue cells on microscopy (saline wet mount), ideally making up at least 20% of epithelial cells.

Three of these four confirm BV. Clue cells and a positive whiff test together are especially strong.

The Nugent score (laboratory, Gram stain)

The Nugent score is the laboratory reference standard and is based entirely on reading a Gram-stained smear of vaginal discharge. It scores the smear by the relative amounts of three bacterial morphotypes:

• Large Gram-positive rods (Lactobacillus): more of these is healthier, so they score down.
• Small Gram-variable rods/coccobacilli (Gardnerella and anaerobes): more of these is more abnormal, scoring up.
• Curved Gram-variable rods (Mobiluncus): scoring up.

The counts convert to a score from 0 to 10:

The logic is intuitive once the frame is clear: the score is really a measure of how far the flora has shifted from Lactobacillus dominance toward Gardnerella and anaerobes. A smear full of large Gram-positive rods is healthy; a smear that has lost them and is full of small Gram-variable organisms is BV.

Why culture is not used

Culturing Gardnerella vaginalis is not recommended for diagnosing BV, because the organism is found in many healthy women without BV. A positive culture would therefore not distinguish disease from normal carriage.

This is a key contrast with organisms whose mere isolation is meaningful, and it follows directly from BV being a shift in the community rather than the presence of one pathogen.

Why bacterial vaginosis matters

BV is common and often causes only discharge and odor, but it is not trivial:

• In pregnancy, BV is associated with an increased risk of preterm labor, premature rupture of membranes, and low birth weight.
• It raises susceptibility to sexually transmitted infections, including HIV, gonorrhea, chlamydia, and herpes, because the loss of the protective acidic flora weakens a natural barrier.
• It is associated with pelvic inflammatory disease and post-procedure infection.

These associations are why BV is treated even when symptoms are mild, particularly in pregnancy.

Treatment

Bacterial vaginosis is treated with antibiotics directed at the overgrowing anaerobes, most commonly metronidazole, with clindamycin as an alternative. The aim is to reduce the anaerobic overgrowth and allow the protective lactobacilli to re-establish. Recurrence is common, because the underlying tendency for the flora to shift is not always corrected by a single course. Specific doses and durations are clinical decisions and are not covered here. Treating male partners has not been shown to prevent recurrence, consistent with BV being sexually associated rather than transmitted.

How to remember

Vaginosis, not vaginitis. The name carries the whole idea. BV is a condition of the flora (-osis), not an inflammation (-itis). That is why there is discharge and odor but little redness or pain, and why the smear has clue cells but few pus cells.

Clue cells give you the clue. A clue cell is a vaginal epithelial cell so covered in adherent bacteria that its border disappears. The name is the mnemonic: it is the clue to BV.

The fishy whiff comes from the anaerobes, and alkali brings it out. The amines that smell fishy become volatile as pH rises, which is why the odor is worse after semen and why adding KOH in the whiff test releases it.

Lost lactobacilli, lost protection. Healthy vagina equals Lactobacillus plus acid plus low pH. BV equals lactobacilli gone, pH up, anaerobes and Gardnerella overgrown. Every finding (high pH, fishy amines, clue cells) flows from that one shift.

Key exam facts

Where students get confused

• BV is a dysbiosis, not a single-organism infection. Gardnerella is the marker, not a lone pathogen. Finding Gardnerella does not by itself mean BV, because it is present in some healthy women too. The diagnosis rests on the flora shift (Amsel or Nugent), not on isolating the organism.
• Vaginosis versus vaginitis. BV causes little inflammation, so the smear shows clue cells with few pus cells. A pus-cell-rich, itchy, inflamed picture points instead toward candidiasis or trichomoniasis. The three causes of abnormal vaginal discharge are compared in the STI overview; this page owns BV itself.
• BV is not a classic STI. It is sexually associated, more common with sexual activity, but it occurs without it and is not transmitted as a single pathogen. Treating the male partner does not prevent recurrence.
• Do not culture to diagnose BV. Because Gardnerella grows from many healthy women, culture cannot separate disease from carriage. Diagnosis is clinical and microscopic.
• Gram-variable, not simply Gram-negative. Gardnerella has a thin Gram-positive-type wall but stains variably, so it is described as Gram-variable. Calling it flatly Gram-negative misses a testable point.

References

1. Tille PM. Bailey and Scott's Diagnostic Microbiology. 15th ed. St. Louis: Elsevier; 2022.
2. Procop GW, Church DL, Hall GS, et al. Koneman's Color Atlas and Textbook of Diagnostic Microbiology. 7th ed. Philadelphia: Wolters Kluwer; 2017.
3. Nugent RP, Krohn MA, Hillier SL. Reliability of diagnosing bacterial vaginosis is improved by a standardized method of gram stain interpretation. J Clin Microbiol. 1991;29(2):297-301. https://doi.org/10.1128/jcm.29.2.297-301.1991
4. Amsel R, Totten PA, Spiegel CA, et al. Nonspecific vaginitis: diagnostic criteria and microbial and epidemiologic associations. Am J Med. 1983;74(1):14-22. https://doi.org/10.1016/0002-9343(83)91112-9
5. Muzny CA, Schwebke JR. Pathogenesis of bacterial vaginosis: discussion of current hypotheses. J Infect Dis. 2016;214(Suppl 1):S1-S5. https://doi.org/10.1093/infdis/jiw121