Dysbiosis of vaginal flora — loss of lactobacilli with overgrowth of anaerobes; thin gray discharge with fishy odor.
Also known as: BV, bacterial vaginosis, Gardnerella, vaginal dysbiosis
Overview
Polymicrobial alteration of the vaginal microbiome characterized by loss of protective Lactobacillus species and overgrowth of anaerobic bacteria including Gardnerella vaginalis, Prevotella, Atopobium vaginae, and Mobiluncus.
Epidemiology
Most common cause of vaginal discharge in reproductive-age women; prevalence ~20-30%. Higher rates in Black and Hispanic women; up to 50% in some populations.
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Question 1ReproductiveEasy
A 22-year-old sexually active woman has thin, gray-white vaginal discharge with a fishy odor. The vaginal pH is 5.2, the potassium hydroxide (KOH) whiff test is positive, and wet prep shows clue cells. Which of the following is the most appropriate treatment?
AOral metronidazole 2 g as a single dose
BOral fluconazole 150 mg as a single dose
COral metronidazole 500 mg twice daily for 7 days
DCeftriaxone 500 mg intramuscularly as single dose
EOral doxycycline 100 mg twice daily for 7 days
Reveal answer & full explanation
Correct answer: C — Oral metronidazole 500 mg twice daily for 7 days
AOral metronidazole 2 g as a single dose
BOral fluconazole 150 mg as a single dose
COral metronidazole 500 mg twice daily for 7 days✓
DCeftriaxone 500 mg intramuscularly as single dose
EOral doxycycline 100 mg twice daily for 7 days
Why oral metronidazole 500 mg twice daily for 7 days is correct
Thin gray-white discharge, fishy odor, pH >4.5 (here 5.2), a positive whiff test, and clue cells satisfy the Amsel criteria for bacterial vaginosis.
The preferred regimen per current CDC STI guidance is oral metronidazole 500 mg twice daily for 7 days, which achieves higher cure rates than a single dose.
Routine partner treatment is not recommended (CDC 2021); in pregnancy, symptomatic BV should be treated, although treatment has not consistently been shown to reduce preterm birth.
Why the others are wrong
Ceftriaxone 500 mg intramuscularly as single dose — treats gonorrhea, not bacterial vaginosis; there is no evidence of gonococcal infection here. Sets the wrong-pathogen trap.
Oral metronidazole 2 g as a single dose — the correct drug but an inferior regimen with a lower cure rate, so it is not preferred for bacterial vaginosis. Sets the right-drug-wrong-regimen trap.
Oral fluconazole 150 mg as a single dose — treats vulvovaginal candidiasis; clue cells and a positive whiff test exclude yeast. Sets the confused-with-candidiasis trap.
Oral doxycycline 100 mg twice daily for 7 days — the twice-daily 7-day course mirrors the correct regimen and is the preferred treatment for chlamydia, but doxycycline is not a recommended therapy for bacterial vaginosis, and clue cells with a positive whiff test point to anaerobic overgrowth rather than chlamydial cervicitis. Sets the look-alike-regimen trap.
Question 2ReproductiveMedium
A 26-year-old woman has thin gray vaginal discharge with a fishy odor. Wet mount shows clue cells and vaginal pH is 5.5. Which of the following best explains the mechanism of this condition?
APseudohyphae from Candida overgrowth
BChlamydial infection of cervical epithelium
CGardnerella invasion of vaginal mucosa
DMotile flagellated protozoan infection
ELactobacilli loss, anaerobic overgrowth
Reveal answer & full explanation
Correct answer: E — Lactobacilli loss, anaerobic overgrowth
APseudohyphae from Candida overgrowth
BChlamydial infection of cervical epithelium
CGardnerella invasion of vaginal mucosa
DMotile flagellated protozoan infection
ELactobacilli loss, anaerobic overgrowth✓
Why Lactobacilli loss, anaerobic overgrowth is correct
Bacterial vaginosis is a dysbiosis: depletion of protective hydrogen-peroxide-producing lactobacilli lets anaerobes such as Gardnerella overgrow.
Loss of lactic acid raises the pH above 4.5 (here 5.5), and amine byproducts create the fishy odor and thin gray discharge.
Clue cells are epithelial cells coated by the overgrown anaerobic bacteria, confirming the mechanism.
Why the others are wrong
Pseudohyphae from Candida overgrowth — Wrong-vaginitis trap: candidiasis gives thick white discharge with a normal pH near 4 and budding yeast, not clue cells.
Motile flagellated protozoan infection — Buzzword-match trap: trichomoniasis shows motile flagellates and frothy yellow-green discharge, and pH overlaps but clue cells point to BV.
Chlamydial infection of cervical epithelium — Wrong-site trap: chlamydial cervicitis produces mucopurulent cervical discharge with a normal vaginal pH and no clue cells or amine odor.
Gardnerella invasion of vaginal mucosa — single-pathogen trap: Gardnerella is the organism most linked to BV and coats the epithelial cells seen as clue cells, but it adheres in a surface biofilm without invading tissue, and BV is a polymicrobial dysbiosis with little inflammation, which is why it is called a vaginosis rather than a vaginitis.
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Disruption of acidic Lactobacillus-dominant microbiome → rise in vaginal pH → overgrowth of facultative and obligate anaerobes producing biofilms and volatile amines (putrescine, cadaverine, trimethylamine) responsible for the characteristic fishy odor. Not classically considered an STI but is sexually associated.
Clinical presentation
Symptoms
Thin, gray-white homogeneous vaginal discharge
Fishy odor, often worse after intercourse or menses
Minimal or no pruritus or inflammation (distinguishes from candidiasis/trichomoniasis)
~50% of cases are asymptomatic
Signs / physical exam
Thin gray-white discharge coating vaginal walls
Absence of significant erythema or edema
No friability
Classic findings
Reproductive-age woman with thin gray discharge, fishy odor after intercourse, vaginal pH >4.5, clue cells on wet mount, positive whiff test.
Differential diagnosis
Vulvovaginal candidiasis — Thick white 'cottage cheese' discharge, intense pruritus, vulvar erythema, normal pH (<4.5); KOH shows yeast/hyphae
Optional commercial molecular tests (BD Affirm, NuSwab) for high-throughput diagnosis
Diagnostic algorithm
Feature
BV
Candidiasis
Trichomoniasis
Discharge
Thin gray-white, homogeneous
Thick white 'cottage cheese'
Frothy yellow-green
Odor
Fishy, worse after sex/menses
None
Malodorous
Pruritus / erythema
Minimal
Marked
Moderate
Vaginal pH
>4.5
<4.5
>4.5
Wet mount
Clue cells
Hyphae/pseudohyphae (KOH)
Motile trichomonads
Whiff test
Positive
Negative
Often positive
First-line Rx
Metronidazole PO or vaginal
Fluconazole 150 mg PO × 1
Metronidazole 500 mg PO BID × 7d
Side-by-side comparison: bacterial vaginosis vs candidiasis vs trichomoniasis.
Treatment
First-line
Metronidazole 500 mg PO BID × 7 days
Metronidazole 0.75% vaginal gel 5 g intravaginally daily × 5 days
Clindamycin 2% vaginal cream 5 g intravaginally at bedtime × 7 days
Pregnancy
Treat all symptomatic pregnant women
Any recommended nonpregnant regimen (oral metronidazole, metronidazole vaginal gel, or clindamycin 2% vaginal cream), or oral clindamycin 300 mg BID × 7 days or clindamycin ovules; avoid tinidazole
Treatment may reduce some adverse pregnancy outcomes
Screening of asymptomatic pregnant women is NOT routinely recommended
Recurrent BV (≥3 episodes/year)
Confirm diagnosis and exclude other causes
Extended initial course followed by suppressive metronidazole gel twice weekly × 4-6 months
Avoid douching, fragranced products
Second-line / adjunct
Tinidazole 2 g PO daily × 2 days or 1 g PO daily × 5 days
Clindamycin 300 mg PO BID × 7 days
Secnidazole 2 g PO × 1 dose
Complications
Increased risk of acquiring/transmitting HIV and other STIs
Increased risk of post-procedural infection (post-hysterectomy cuff cellulitis, post-abortion endometritis)
In pregnancy: associated with preterm birth, PROM, chorioamnionitis, postpartum endometritis (causal role debated)
Increased PID risk (modest)
PANCE pearls
BV is not classified as an STI; CDC 2021 does not recommend routine treatment of male partners, but the 2025 StepUp RCT found that treating monogamous male partners with oral metronidazole plus topical clindamycin cream reduced recurrence (about 35% vs 63%), an emerging option for recurrent BV.
Classic teaching and FDA labeling warn of a disulfiram-like reaction with alcohol during oral metronidazole or tinidazole, but CDC 2021 states that abstaining from alcohol is unnecessary.
Asymptomatic BV generally does not require treatment; screening and treatment can be considered before hysterectomy (ACOG), and BV screening is not required before IUD insertion.
Clindamycin cream is oil-based and weakens latex condoms — use alternative contraception during and 5 days after treatment.
Recurrence is common (>50% within 6-12 months) — counsel about realistic expectations.
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