A deeper understanding of the bacteria in BV and how they interact

Bacterial vaginosis (BV) is a polymicrobial infection​1​, with a few key players, many of which are antibiotic-resistant. Gardnerella vaginalis is the most often cited as main player in BV​2​. BV is a state of vaginal microbial dysbiosis​3​ (imbalanced microbes).

G. vaginalis​4​ is found in about 95 per cent of those with BV in varying amounts, but it is also found in up to 50 per cent of those without BV. It is also found in the digestive systems of men and children.

G. vaginalis is known to make biofilms, which help support other bacteria that contribute to BV. High levels of G. vaginalis in the vaginal tract is an indicator of BV​5​. The G. vaginalis biofilm is resistant to antibiotics, making BV often recurrent.

Bacteria implicated in BV include:​6​

The BV microbial landscape​7,8​

Normal, and then intermediate, then full-blown BV vaginal flora was studied, and at the intermediate stage, G. vaginalis and Bacteroides were present in moderate amounts.

High concentrations of those microbes and M. hominis were not seen until BV had fully developed, indicating that the environment set up by G. vaginalis and Bacteroides was suitable for the new bacteria to colonise.

Gram stains have been used to define abnormal flora (using Nugent scores of 9 or 10). Polymerase chain reaction (PCR) testing (DNA/RNA testing) has detected Mobiluncus spp. in almost 85 per cent of women with BV and 38 per cent of those without BV.

One bacteria produces the food for the others, and vice versa

These disruptive bacteria are anaerobic, and produce succinic acid rather than lactic acid. (Lactobacilli are acid-loving bacteria and produce lactic acid.)

G. vaginalis and Prevotella bivia have a symbiotic relationship whereby G. vaginalis produces amino acids that are then utilised by P. bivia. P. bivia then produces ammonia, which is then used by G. vaginalis.

Additionally, researchers found that P. bivia made amino acids that were also made available for Peptostreptococcus anaerobius​9​.

The BV-related bacteria also break down cervical and vaginal mucous in the vagina using enzymes – these are mucinases, sialidases, and neuraminidases. This is the likely reason why the discharge in BV changes.

Immune system protections (immunoglobulin A IgA) and IgM) are cleaved (broken into like you would crack open an oyster shell) by certain factors in the vagina produced by the bacteria (known as virulence factors), reducing the capacity of the vagina to defend itself. Secretory leukocyte protease inhibitor – SLPI – is also reduced.​10​

How lactobacilli fit in​11–14​

Lactobacilli also produce substances that are toxic to other bacteria including some other lactobacilli species. An acidic vagina is a great defence, along with the hydrogen peroxide produced, which also helps protect against sexually transmitted infections entering the vagina.

A low pH inhibits BV-associated bacteria effectively, but higher pH levels cause the positive effect to quickly wane. A special substance is produced by BV-related microbes, myeloperoxidase, which breaks down hydrogen peroxide, further weakening lactobacilli.

Menstrual blood and semen both change the pH to less acidic, and possibly contribute to the change in flora and BV development.

Bacteria and pH​15​

Common lactobacilli strains in the vagina include Lactobacillus crispatus, followed by L. jensenii and L. gasseri.

In vitro, the rate of acid production by Lactobacillus spp. was measured, keeping the vagina at a pH of between 3.2 and 4.8.

BV bacteria like G. vaginalis, P. bivia, and Peptostreptococcus anaerobius reached asymptotic levels at a pH of between 4.7 and 6.0. This is the sort of pH we expect to see in BV.

Semen, pH and BV​16–21​

Semen was tested – it was determined that 3ml of semen would be acidified at a rate of 0.56 to 0.75 pH units per hour.

Questions around semen include, would one incident of unprotected sex cause BV by changing the acidity for a short period of time? We don’t know, but having several sex sessions over a 24-hour period may weaken the vaginal flora’s defences enough, if it was already weakened.

  • Normal semen volume: 1.5 – 3.7ml per ejaculate
  • Normal semen pH range: 7.1 – 8.0
  • Acidification rate: 3ml of semen is acidified at a rate of 0.56 – 0.75 pH units per hour


  1. 1.
    Hay PE, Brogden KA, Guthmiller J. Bacterial Vaginosis as a Mixed Infection. Polymicrobial Diseases. Washington (DC): ASM Press; 2002. Chapter 7. Published 2002. https://www.ncbi.nlm.nih.gov/books/NBK2495/
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    Morrill S, Gilbert NM, Lewis AL. Gardnerella vaginalis as a Cause of Bacterial Vaginosis: Appraisal of the Evidence From in vivo Models. Front Cell Infect Microbiol. Published online April 24, 2020. doi:10.3389/fcimb.2020.00168
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Jessica Lloyd - Vulvovaginal Specialist Naturopathic Practitioner, BHSc(N)

Jessica is a degree-qualified naturopath (BHSc) specialising in vulvovaginal health and disease, based in Melbourne, Australia.

Jessica is the owner and lead naturopath of My Vagina, and is a member of the:

  • International Society for the Study of Vulvovaginal Disease (ISSVD)
  • International Society for the Study of Women's Sexual Health (ISSWSH)
  • National Vulvodynia Association (NVA) Australia
  • New Zealand Vulvovaginal Society (ANZVS)
  • Australian Traditional Medicine Society (ATMS)