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Not all lactobacilli are good for

vaginal health

Lactobacilli are the most prevalent microorganisms in a healthy vaginal microbiota.
Changes in the composition of the vaginal microbiota can weaken its protective capacity and increase susceptibility to infections. Earlier studies [1] have demonstrated that in women with chlamydia, the vaginal microbiota is low in lactobacilli and high in a range of anaerobic bacteria, as seen in bacterial vaginosis. However research shows that not all lactobacilli are beneficial for vaginal health. 

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In the majority of healthy fertile women, the vagina is populated by lactic acid bacteria, or lactobacilli. They offer protection against pathogens by synthesizing lactic acid and other antimicrobial substances. Although there are hundreds of Lactobacillus species known to science, the vaginal microbiota is typically dominated by a mere four: 


  • Lactobacillus crispatus
  • Lactobacillus gasseri
  • Lactobacillus iners
  • Lactobacillus jensenii
The vaginal microbiota is typically dominated by a mere four types of lactobacilli



Lactobacillus iners

To increase scientific understanding in this area, researchers in Amsterdam looked at the composition of the vaginal microbiota before and after a chlamydia infection. Their research involved a total of 122 female subjects aged between 16 and 29 attending an STI clinic in Amsterdam [2].

In the Netherlands, chlamydia is one of the most common sexually transmitted infections. The bacterium Chlamydia trachomatis embeds itself in the mucosae and causes an infection. Infections often remain symptom-free but can lead to infertility and increase the likelihood of contracting an HIV infection if left untreated.[3] 


After one year, half of the participants in the Amsterdam study had contracted a chlamydia infection. Women whose vaginal microbiota was dominated by L. iners before the infection were shown to be at an increased risk of a chlamydia infection. In women who had remained chlamydia-free, vaginal microbiotas were more often seen to be dominated by L. crispatus.





The odd one out

What is sometimes overlooked is that, although classed as one of the lactic acid bacteria, Lactobacillus iners is in fact the outsider among them. It is highly dissimilar not only to other vaginal lactobacilli but even to all other known lactic acid bacteria, and stands out because of, for instance, its much smaller size and its genome, which amounts to only about a quarter of that of some other lactobacilli. This is indicative of a parasitic or symbiotic biology compatible with conditions in the vagina [4], which makes it difficult for researchers to culture and study this Lactobacillus species in the lab. That is probably one of the reasons it was named iners, which is the Latin word for sluggish or lazy [5]. It does produce some, but very little lactic acid, making it an ineffective acidifier. In contrast to the well-known probiotic lactobacilli, L. iners is capable of producing a toxin that is similar to the one produced by the vaginal pathogen Gardnerella vaginalis

The vaginal lactic acid bacterium species Lactobacillus iners increases susceptibility to chlamydia infection, revealing L. iners to be the odd one out.



All of this means the role of Lactobacillus iners in vaginal health remains unclear to this day, because it has a dominant presence both in normal conditions and during vaginal dysbiosis. Moreover, it seems to offer little protection against vaginal dysbiosis, and possibly even to increase the risk of developing it. This may also explain the increased risk of sexually transmitted infections and miscarriages presented by a vaginal microbiota where L. iners is dominant.[6] How specific Lactobacillus species in the vaginal microbiota contribute to increased susceptibility to or protection against infections is a matter that deserves to be studied further. 




1.       Brotman RM, Klebanoff MA, Nansel TR, et al. Bacterial vaginosis assessed by gram stain and diminished colonization resistance to incident gonococcal, chlamydial, and trichomonal genital infection. J Infect Dis 2010;202:1907–15.

2.       van Houdt R, Ma B, Bruisten SM, Speksnijder AGCL, Ravel J and de Vries HJC, Lactobacillus iners-dominated vaginal microbiota is associated with increased susceptibility to Chlamydia trachomatis infection in Dutch women: a case- control study, Sexually Transmitted Infections, Published Online First: September 25, 2017. doi:10.1136/sextrans-2017-053133.

3. (Visited in November 2017) 4.       Petrova MI, Reid G, Vaneechoutte M, and Lebeer S, Review Lactobacillus iners: Friend or Foe? Trends in Microbiology, March 2017, Vol. 25, No. 3

5.       Falsen E, Pascual C, Sjödén B, Ohlén M and Collins MD, Phenotypic and phylogenetic characterization of a novel Lactobacillus species from human sources : description of Lactobacillus iners sp.nov. International Journal of Systematic Bacteriology (1 999), 49, 21 7-221

6.       Vaneechoutte M, Lactobacillus iners, the unusual suspect, Res Microbiol. 2017 Sep 22. pii: S0923-2508(17)30151-1. doi: 10.1016/j.resmic.2017.09.003. validation using human small intestine microbiota datasets. BMC Genomics 14:530.




Not all lactobacilli are good for vaginal health

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