Bacterial vaginosis in adults

Bacterial vaginosis (BV) is the most common cause of vaginal discharge in women of reproductive age, accounting for approximately 40–50% of cases presenting to primary care and sexual health clinics. BV is characterised by a disruption of the normal vaginal microbiome, with replacement of the dominant Lactobacillus species by an overgrowth of anaerobic and facultative bacteria, most notably Gardnerella vaginalis, Prevotella spp., Mobiluncus spp., Mycoplasma hominis, and Atopobium vaginae (now classified as Fannyhessea vaginae).

In Australia, the prevalence of BV among women of reproductive age is estimated at 15–30%, with higher rates in certain populations including Aboriginal and Torres Strait Islander women (up to 40–50% in some community studies), women who have sex with women (WSW), and women with multiple sexual partners. BV is not classified as a sexually transmitted infection, but sexual activity — particularly new or multiple partners, receptive vaginal sex without condoms, and sex with women — is strongly associated with disruption of the vaginal microbiome and BV recurrence.

The healthy vaginal microbiome is dominated by Lactobacillus species (predominantly L. crispatus and L. iners), which produce lactic acid and hydrogen peroxide, maintaining a low vaginal pH (<4.5) and inhibiting pathogen overgrowth. In BV, this protective ecosystem is disrupted by a polymicrobial overgrowth of anaerobes, leading to elevated vaginal pH (>4.5), reduced lactobacilli, and production of amines (cadaverine, putrescine, trimethylamine) responsible for the characteristic fishy odour.

Key Organisms in BV

• Gardnerella vaginalis: Most consistently isolated organism; forms the initial biofilm on vaginal epithelium. Present in up to 95% of BV cases. Highly adherent, produces cytotoxin (vaginolysin). Biofilm formation is central to BV pathogenesis and recurrence.
• Fannyhessea (Atopobium) vaginae: Strongly associated with BV and recurrence. Embedded within G. vaginalis biofilm. Associated with treatment failure with metronidazole. Sensitive to clindamycin.
• Anaerobes: Prevotella spp., Mobiluncus spp., Peptostreptococcus spp., Bacteroides spp. — produce amines and contribute to elevated pH and inflammation.
• Mycoplasma hominis: Frequently co-isolated; contributes to upper genital tract complications.
• Disrupting factors: Douching, new or multiple sexual partners, semen exposure (alkaline pH disrupts lactobacilli), smoking, intrauterine devices (IUDs), and antibiotic use.

Symptoms

• Vaginal discharge: Thin, homogeneous, greyish-white, adherent to vaginal walls. Increased quantity compared to normal.
• Fishy odour: Characteristic malodour, often more prominent after sexual intercourse (semen raises pH, volatilising amines) or during menstruation.
• Absence of significant inflammation: BV does not typically cause vaginal erythema, significant pruritus, or soreness — distinguishing it from candidiasis and trichomoniasis.
• Asymptomatic BV: Up to 50% of women with BV are asymptomatic. Incidental diagnosis is common on routine cervical screening or STI testing.

Amsel Criteria (Clinical Diagnosis)

BV is clinically diagnosed using the Amsel criteria — three of four criteria must be met:

Nugent Score (Gram Stain — Laboratory Diagnosis)

The Nugent score is the laboratory gold standard for BV diagnosis, applied to a Gram-stained vaginal smear. Score 0–3 = normal; 4–6 = intermediate; 7–10 = BV. A score ≥7 is diagnostic of BV. Used in research and laboratory confirmation.

• Essential
  Vaginal pH measurement

  Apply pH paper to discharge on speculum blade (avoid contamination with cervical mucus which is alkaline). pH >4.5 is consistent with BV, trichomoniasis, or atrophic vaginitis. pH <4.5 virtually excludes BV. Simple, cheap, and available in all primary care settings. Most sensitive Amsel criterion.
• Essential
  Wet mount microscopy

  Low-power (×10) and high-power (×40) microscopy of fresh vaginal secretions in normal saline. Identifies clue cells (≥20% = BV), motile trichomonads, and hyphae/pseudohyphae (candida). Most specific point-of-care test. Requires trained operator and immediate examination of fresh specimen.
• Available
  Nugent score (Gram-stained vaginal smear)

  Laboratory gold standard. Gram stain of vaginal swab scored for Lactobacillus morphotypes (large Gram-positive rods), G. vaginalis morphotypes (small Gram-variable rods), and curved Gram-negative rods (Mobiluncus). Score 7–10 = BV. Available at all Australian pathology laboratories. Useful for confirmation in research settings or ambiguous cases.
• Available
  NAAT-based BV testing

  Molecular assays (e.g., SureSwab BV, Aptima BV) detect BV-associated organisms including G. vaginalis, F. vaginae, and BVAB-2. High sensitivity and specificity; available at major Australian pathology providers. Particularly useful in asymptomatic BV and research settings. More expensive than clinical criteria.
• Essential
  STI co-testing

  NAAT for Chlamydia trachomatis, Neisseria gonorrhoeae, and Trichomonas vaginalis (self-collected vaginal swab). Syphilis serology. HIV serology. BV frequently co-exists with STIs. T. vaginalis NAAT is more sensitive than wet microscopy and should be performed if TV is clinically suspected.
• Available
  Point-of-care pH + amine (OSOM BV Blue)

  Rapid point-of-care test detecting elevated pH and proline iminopeptidase activity (marker of anaerobic bacteria). Sensitivity ~90%, specificity ~60% vs. Nugent. Useful in resource-limited settings where microscopy is unavailable. Available in some Australian clinics and remote health services.

First-Line Treatment — Non-pregnant Adults

Alternative Treatment

Recurrent BV — Suppressive Therapy

Metronidazole Treatment Failure

• Switch to clindamycin: If initial metronidazole course fails (symptoms persist or recur within 3 months), switch to clindamycin vaginal cream 2% × 7 days or oral clindamycin 300 mg BD × 7 days. F. vaginae (commonly present in biofilm) is metronidazole-resistant but clindamycin-sensitive.
• Combination therapy (recurrent): Combined oral metronidazole 400 mg BD × 7 days + intravaginal clindamycin cream × 7 days has been used in recurrent BV — limited evidence but may address mixed biofilm.
• Boric acid suppositories: Intravaginal boric acid 600 mg daily for 14 days is used as adjunctive therapy for recurrent BV and mixed BV/candida infections. Not PBS listed; available at compounding pharmacies. Toxic if ingested — must be stored safely. Not for use in pregnancy.
• Reassess diagnosis: If recurrent or persistent, reassess — exclude STIs, consider desquamative inflammatory vaginitis (DIV), aerobic vaginitis, or lichen planus. Refer to gynaecology or sexual health specialist.

Addressing Biofilm and Recurrence

• Biofilm disruption: G. vaginalis biofilm is a key driver of recurrence. Some clinicians use intravaginal boric acid or antiseptic preparations (e.g., dequalinium chloride) as biofilm-disruptive adjuncts — limited Australian guideline evidence; consider under specialist guidance.
• Vaginal acidification: Lactic acid/citric acid vaginal gels (e.g., RepHresh, Balance Activ) can help maintain vaginal pH. Not a first-line treatment but may reduce recurrence when used as maintenance after successful BV treatment.
• Probiotics: Oral or vaginal Lactobacillus-based probiotics may modestly reduce BV recurrence; evidence remains limited. L. rhamnosus and L. reuteri strains have the most supportive data. Not recommended as standalone treatment.
• Partner treatment: Routine treatment of male partners does not reduce BV recurrence and is not recommended. For women who have sex with women (WSW), concurrent treatment of female partner(s) may reduce recurrence — limited evidence; discuss with patient.

BV is managed entirely with oral or intravaginal therapy in outpatient settings. IV treatment is not required for uncomplicated BV. Parenteral metronidazole may be used in the context of co-existing pelvic inflammatory disease or post-procedural pelvic infection requiring IV antimicrobials.

• Post-procedural pelvic infection: If BV is identified alongside post-surgical or post-instrumentation infection, IV metronidazole (500 mg 8-hourly) in combination with appropriate cover for aerobic organisms is used until clinical improvement, then switch to oral metronidazole 400 mg BD to complete 14 days total.
• Switch criteria: Afebrile, clinically improving, tolerating oral intake, no signs of pelvic sepsis.

🤰 Pregnancy

BV in pregnancy is associated with preterm birth, late miscarriage (second trimester), preterm prelabour rupture of membranes (PPROM), low birth weight, and postpartum endometritis. Treatment is recommended for all symptomatic pregnant women and for asymptomatic women with prior preterm birth.

• First trimester: Oral metronidazole 400 mg BD × 7 days. Metronidazole has an extensive safety record in pregnancy; evidence does not support teratogenicity. Previously avoided in first trimester due to theoretical concerns — current Australian and international guidelines support use throughout pregnancy. Avoid single high-dose (2 g) regimens in pregnancy.
• Second/third trimester: Oral metronidazole 400 mg BD × 7 days or metronidazole vaginal gel × 5 days. Clindamycin vaginal cream in second/third trimester is associated with increased risk of low birth weight and neonatal infection in some studies — use with caution; oral clindamycin is preferred if clindamycin needed in pregnancy.
• Preterm birth prevention: Screening and treating BV in women with a prior preterm birth is recommended. Evidence for routine screening of low-risk pregnancies is limited — discuss with obstetrician.
• Test of cure: Repeat vaginal assessment 1 month post-treatment in pregnancy.

👩‍❤️‍👩 Women Who Have Sex with Women (WSW)

WSW have substantially higher rates of BV than women who have sex with men exclusively. Vaginal microbiome sharing between partners is associated with concordant BV, and recurrence is common if both partners are not addressed.

• Partner treatment: While evidence is limited, concurrent treatment of female sexual partners may reduce recurrence in WSW with recurrent BV. Discuss with the patient.
• STI screening: WSW may be at risk of STIs transmitted via genital secretions (chlamydia, HPV, HSV, BV, trichomoniasis). Ensure full STI screening.
• Dental dams and barrier methods: Counsel on use of dental dams and other barriers to reduce vaginal microbiome transmission.

🛡️ Immunocompromised

Immunocompromised women (including those on immunosuppressants, chemotherapy, or with HIV) may have more severe, recurrent, or treatment-resistant BV.

• HIV-positive women: BV is more prevalent and more recurrent in HIV-positive women. BV increases HIV shedding in vaginal secretions. Treat with standard regimens; higher recurrence rates should be anticipated. More frequent STI screening recommended.
• Treatment: Standard metronidazole or clindamycin regimens. Suppressive maintenance therapy may be required for recurrent BV in immunocompromised patients.

🫘 Renal and Hepatic Impairment

Metronidazole is hepatically metabolised; dose reduction is required in severe hepatic impairment. Clindamycin may be preferred.

• Severe hepatic impairment (Child-Pugh C): Reduce metronidazole dose — consult pharmacist. Consider clindamycin vaginal cream as alternative (minimal systemic absorption).
• Renal impairment: Oral metronidazole — no dose adjustment required for mild–moderate CKD. Accumulation of metabolites in severe renal failure (eGFR <10) — consult pharmacist for prolonged courses.

👴 Postmenopausal Women

Oestrogen deficiency post-menopause leads to reduced lactobacilli and elevated vaginal pH, predisposing to recurrent BV and aerobic vaginitis.

• Treatment: Standard metronidazole or clindamycin regimens. Consider vaginal oestrogen cream as adjunct to restore Lactobacillus-dominant microbiome and reduce recurrence in postmenopausal women.
• Differential: Atrophic vaginitis often co-exists — consider pH, microscopy findings, and clinical context.

• Confirm diagnosis before treating: Do not treat vaginal discharge empirically with antibiotics without clinical or laboratory confirmation of BV. Empirical treatment leads to overuse and may worsen candidiasis or mask other diagnoses.
• Avoid treating asymptomatic BV in non-pregnant women: Routine treatment of asymptomatic BV in non-pregnant women without upcoming procedures is not indicated based on current evidence. Treat symptomatic women and those at risk of complications.
• Restrict oral clindamycin: Oral clindamycin is associated with Clostridioides difficile colitis. Use intravaginal clindamycin cream preferentially over oral systemic therapy where topical route is feasible. Reserve oral clindamycin for metronidazole failures or contraindications.
• Single-dose metronidazole (2 g): Avoid single-dose 2 g oral metronidazole except where 7-day adherence is very unlikely — it has lower cure rates (~80% vs. ~90% for 7-day course). The short-term convenience does not justify reduced efficacy.
• No role for partner treatment in male partners: Routine antibiotic treatment of male partners does not improve cure rates or reduce recurrence — do not prescribe without specific indication.

Follow-Up Guidance

Prevention and Self-Care

• Avoid douching: Vaginal douching disrupts the microbiome and is strongly associated with BV recurrence. Advise women to avoid all intravaginal washing, deodorants, and scented products.
• Condom use: Consistent condom use is associated with reduced BV incidence and recurrence (reducing alkaline semen exposure).
• Smoking cessation: Smoking is associated with BV recurrence — advise cessation and offer referral to Quitline (13 7848).
• Vaginal pH maintenance: Lactic acid vaginal gel (e.g., Balance Activ, RepHresh) used after menstruation or sex may help maintain low vaginal pH and reduce recurrence. Not a substitute for antibiotic treatment.
• Probiotics: Oral or vaginal Lactobacillus probiotics (e.g., L. rhamnosus GR-1 + L. reuteri RC-14) as adjuncts may modestly reduce recurrence. Can be recommended as a low-risk adjunct alongside standard treatment.

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