Bacterial Vaginosis Gardnerella Vaginalis

After an STD test, it is surprisingly common to hear "this is bacterial vaginosis." If the discharge has a fishy smell even though itchy and stinging symptoms are not as distinct as with fungal vaginitis (candida) or trichomonas, behind it is often a bacterium called Gardnerella (Gardnerella vaginalis). Though grouped under the same name "vaginitis," bacterial vaginosis is closer to a disease that differs in causative bacteria, diagnostic criteria, treatment response, and recurrence pattern. In this article, centered on Gardnerella and biofilm, which could not be fully covered in other vaginitis articles, I will organize everything from why the smell occurs to why it keeps recurring.

Bacterial vaginosis is, rather than a simple infection caused by a single pathogen entering from outside, a state in which the balance of the vaginal microbial ecosystem has collapsed. In a healthy vagina, Lactobacillus is dominant and makes lactic acid and hydrogen peroxide to maintain an acidic environment, and when this Lactobacillus decreases, anaerobic bacteria fill the vacancy. The US Centers for Disease Control and Prevention (CDC, 2021) defines bacterial vaginosis as a state in which Lactobacillus has been "replaced" by various anaerobic bacteria such as Gardnerella, Prevotella, and Mobiluncus.

The key here is that Gardnerella, rather than causing symptoms alone, grows in a complex manner together with several anaerobic bacteria. As noted in the original article, because complex infection is more common than single infection, the picture is not completed by targeting just one bacterium.

The most characteristic signal of bacterial vaginosis is grayish-white thin discharge and a fishy odor like fish. There is a clear biochemical reason for this smell. As anaerobic bacteria break down amino acids, they produce volatile amines (putrescine, cadaverine, etc.), and these amines fly off into the air and give off the characteristic fishy smell. In particular, when alkaline substances such as semen or soap touch, the amines evaporate more actively, so it is reported that the smell tends to worsen after intercourse or around menstruation.

The enzymes Gardnerella produces also play an important role. Some strains of Gardnerella secrete an enzyme called sialidase that breaks down the protective mucus layer covering the vaginal mucosa, and this process becomes the foothold for the bacteria to adhere more firmly to epithelial cells and form a biofilm.

The symptoms can be organized as follows.

• Grayish-white or light gray thin discharge
• A fishy odor like fish, worsening after intercourse or around menstruation
• Itching or burning is often weak or absent

If, on the contrary, itchy and stinging symptoms are prominent, you must look together at the possibility of other vaginitis such as candida. If the discharge pattern is confusing, reading together the criteria for distinguishing the normal range from vaginitis is helpful.

Because the symptoms of bacterial vaginosis are nonspecific, it is hard to conclude by clinical appearance alone. So we use objective criteria. The Amsel criteria, widely used in practice, regard it as bacterial vaginosis if three or more of four items are satisfied.

A representative finding is the appearance, under the microscope, of clue cells with bacteria densely adhering to the surface of epithelial cells, and the Nugent score is known as the standard method of scoring bacterial morphology by Gram stain. As with the 12-type STD test mentioned in the original article, using molecular diagnosis together helps grasp which anaerobic bacteria are growing together, not Gardnerella alone. If you are curious about testing, you can refer to the article organizing what the 12-type STI test is.

If the diagnosis is ambiguous or symptoms recur, it is better, rather than self-judgment, to check the basics you should know about vaginal discharge and receive medical care.

The standard treatment for bacterial vaginosis is antibiotics. The CDC (2021) recommends oral metronidazole, metronidazole vaginal suppositories, and clindamycin vaginal cream as first-line treatment. As noted in the original article, both oral and intravaginal administration of the same component are used, and the medical staff choose according to symptoms and situation. Since the exact dose and duration have individual differences, it is safe to receive guidance after a visit.

During treatment, avoid alcohol (this is recommended when taking metronidazole), and douching is not recommended because it can rather break down the balance further and raise the risk of recurrence. One frequently received question is "should the partner be treated together as well?" The CDC, considering that simultaneous treatment of a typical heterosexual partner does not change the woman's treatment response or recurrence rate, does not generally recommend it.

If you arbitrarily stop the drug just because symptoms have disappeared, the bacteria may remain without being sufficiently reduced, so it is important to complete the prescribed period to the end.

The most frustrating part of bacterial vaginosis is the high recurrence rate. Even with good treatment, a considerable number are reported to experience symptoms again within a few months, and the core background of this is precisely the biofilm that Gardnerella forms.

A biofilm is a structure in which bacteria gather inside a sticky protective film and adhere to the epithelium. This film makes antibiotic penetration difficult, so it is known that even if the bacteria on the surface decrease, the bacteria hidden inside the film survive and then proliferate again. In other words, recurrence often happens not "because it did not heal" but "because anaerobic bacteria settled in again before the collapsed ecosystem fully recovered to Lactobacillus dominance."

In cases of repeated recurrence, the medical staff consider approaches such as the following.

• Keeping the bacteria low with suppressive therapy for a certain period
• Checking habits that disturb the balance, such as douching and strongly scented cleansers
• Combining management that helps restore the Lactobacillus environment

In clinical experience, there is no need to be discouraged just because it recurred once or twice. Watching the recurrence pattern together and adjusting the management strategy is the realistic solution. If it is a recurring pattern, we recommend looking together at how to manage chronic vaginitis.

The reason it is hard to dismiss bacterial vaginosis as merely "a discomfort with a bit of smell" is that the defensive environment within the vagina is in a weakened state. The CDC (2021) explains that when there is bacterial vaginosis, one can become more vulnerable to sexually transmitted infections including HIV, gonorrhea, chlamydia, and trichomonas. If pregnant, an association with complications such as premature rupture of membranes, preterm birth, and postpartum infection is also reported.

Therefore, if a fishy smell or change in discharge recurs, rather than dismissing it as "it must just be vaginitis," it is desirable to confirm the causative bacteria and treat appropriately to reduce the risk of additional infection and transmission. As emphasized in the original article, diagnosing and treating in advance not only reduces your own discomfort but is also meaningful in preventing the spread of sexually transmitted infections.

If symptoms keep recurring or changes in smell or discharge bother you, please do not worry alone but receive medical care.

Written by: Lee Dong-hee Director · Obstetrics and Gynecology Specialist · View medical staff profile

First published June 7, 2024 · Last reviewed May 30, 2026

References: CDC STI Treatment Guidelines (2021), American Journal of Medicine — Amsel Criteria (1983)

This article is intended to provide general health information and does not replace individual diagnosis or treatment. If you have symptoms, please consult through a medical visit.