The Vaginal Microbiome
Lactobacillus dominance, community state types, and clinical implications.
What's covered
- 01Community state types (CSTs) I–V
- 02Lactobacillus crispatus vs L. iners — not all Lactobacillus are equal
- 03Bacterial vaginosis: polymicrobial, Gardnerella-driven dysbiosis
- 04Vaginal microbiome and STI susceptibility (HIV, HPV)
- 05Pregnancy outcomes and preterm birth risk
- 06Racial and ethnic variation in CST prevalence
By the end of this module you will be able to
- L01Describe the five community state types and their clinical significance.
- L02Explain why L. crispatus is considered protective while L. iners is ambiguous.
- L03Outline the relationship between BV, increased STI risk, and preterm birth.
- L04Discuss the implications of racial variation in CST prevalence for clinical research.
What you should walk away believing
- →A 'healthy' vaginal microbiome is Lactobacillus-dominant — but not all Lactobacillus species are equal.
- →BV is not a simple infection; it's a polymicrobial dysbiosis that recurs because the biofilm persists.
- →CST-IV (diverse, non-Lactobacillus) is more common in Black and Hispanic women — research frameworks must account for this without pathologizing normal variation.
What this means for you
A healthy vagina is normally dominated by Lactobacillus bacteria that produce lactic acid, keeping the environment acidic and protective. When this balance is disrupted — by antibiotics, douching, or other factors — it can lead to bacterial vaginosis, which increases the risk of STIs and pregnancy complications. Not all disruptions cause symptoms, and not all need treatment.
Classify vaginal microbiomes by CST: I (L. crispatus, most protective), II (L. gasseri), III (L. iners, transitional/ambiguous), V (L. jensenii), IV (diverse anaerobes, BV-associated). L. iners (CST-III) can coexist with BV-associated bacteria and may facilitate transition to CST-IV. BV recurrence (~50% at 12 months post-antibiotics) is driven by Gardnerella biofilm persistence — antibiotics suppress but rarely eradicate.
Lactic acid exists in D and L isomers; L. crispatus produces both, while L. iners produces primarily the less-protective L-isomer. D-lactic acid has stronger HIV virucidal activity. Multi-omic studies (metabolomics + metagenomics) are revealing that functional output matters more than taxonomic composition — two women with similar 16S profiles can have different metabolomic landscapes and clinical outcomes.
Douching helps maintain vaginal health.
Douching disrupts the Lactobacillus-dominant community and is associated with increased BV, STI, and PID risk. Every major gynecological society advises against it.
Recurrent BV in a 28-year-old
A 28-year-old woman presents with her fourth episode of bacterial vaginosis in 12 months. Each episode resolved with metronidazole but recurred within 8 weeks. She asks about vaginal probiotic suppositories she saw advertised on Instagram and wants to know about 'vaginal microbiome transplant.'
How would you explain BV recurrence mechanisms, evaluate the evidence for vaginal probiotics, and counsel on the current state of vaginal microbiome transplantation?
What the data says
Test yourself
Spaced review
Key terms & abbreviations
- Community state typeCST
- Classification of vaginal microbiome composition based on dominant Lactobacillus species or diverse anaerobic community.
- Bacterial vaginosisBV
- Polymicrobial dysbiosis characterized by depletion of Lactobacillus and overgrowth of anaerobes including Gardnerella.
Optional deeper dive
- Vaginal microbiome of reproductive-age women — Ravel J et al., PNAS 2011↗