What are the most common bacteria found in the female vagina?

Most Common Bacteria in the Female Vagina

In healthy reproductive-age women, Lactobacillus species—particularly L. crispatus, L. iners, L. gasseri, and L. jensenii—are the dominant vaginal bacteria, with these hydrogen peroxide-producing lactobacilli maintaining vaginal health through acidification and competitive exclusion of pathogens. 1, 2, 3

Normal Vaginal Flora in Reproductive-Age Women

The healthy vaginal microbiome is characterized by:

• Lactobacillus species as predominant bacteria (contributing approximately 64% of vaginal bacterial composition on average), specifically:

  • Lactobacillus crispatus 4, 5, 6
  • Lactobacillus iners (notably the most common species identified by molecular methods, though often missed by traditional culture) 4, 5, 6
  • Lactobacillus gasseri 4, 7
  • Lactobacillus jensenii 7, 6

• Secondary normal flora includes staphylococci, diphtheroids, streptococci, and micrococci in decreasing order of frequency 8

• Gram-negative enterobacteria are rare in healthy women (present in only 6-9% of normal vaginal flora), and when present, colonization is typically transitory 8

Pathological Bacterial Overgrowth

When the normal H₂O₂-producing Lactobacillus species are replaced, bacterial vaginosis (BV) develops with:

• High concentrations of anaerobic bacteria including:

  • Prevotella species 1, 2, 3
  • Mobiluncus species 1, 2, 3
  • Bacteroides species (including B. fragilis) 2, 4

• Other BV-associated organisms:

  • Gardnerella vaginalis (present in high concentrations, though can be isolated from 50% of normal women at low levels) 1, 2, 3, 6
  • Mycoplasma hominis 1, 2, 3
  • Atopobium vaginae 6, 9
  • Ureaplasma urealyticum 9

Age-Related Variations

Postmenopausal women demonstrate more complex vaginal bacterial communities compared to reproductive-age women, with common bacteria including:

• Lactobacillus iners and L. crispatus (still present but less dominant) 4
• Escherichia coli 4
• Streptococcus species (S. agalactiae, S. gallolyticus, S. intermedius, S. anginosus) 4
• Prevotella species 4
• Bacteroides fragilis 4
• Veillonella species and Anaerococcus lactolyticus 4

Clinical Significance

The dominance of Lactobacillus species maintains vaginal pH <4.5, which is protective against pathogenic overgrowth 3. When this balance is disrupted and vaginal pH rises above 4.5, it indicates loss of lactobacilli dominance and potential dysbiosis 3.

Over 250 bacterial species have been detected in the vagina using genomic sequencing, but the healthy state is paradoxically characterized by low diversity with lactobacilli dominance 5. Increased diversity typically signals dysbiosis rather than health in this unique anatomical niche 5, 9.