What role does L. iners play in vaginal health during pregnancy?
Lactobacillus iners, found in both healthy and dysbiotic microbiota, plays an ambiguous role that a study in pregnant women has shed new light on.
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Diversity in vaginal microbiota profiles
Vaginal microbiota is made up of five different community state types (CSTs), including three protective CSTs (respectively dominated by Lactobacillus crispatus, Lactobacillus gasseri or Lactobacillus jensenii), one disruptive CST, and one—CST III—dominated by L. iners, whose role in vaginal health remains debated. This is because the bacterium may be protective or disruptive—while it seems to be an integral part of a healthy vaginal microbiome, it is paradoxically also abundant in pathological and dysbiotic conditions and has even been implicated in colonization by Group B Streptococcus during pregnancy.
More L. iners in healthy expectant mothers
To better understand the relationship between microbial flora and vaginal health, Chinese researchers focused on the particular case of pregnant women in their third trimester—either healthy (34 women) or not (61 women with gestational diabetes, complications, infection, etc.). Their results highlight the persistence of lactobacilli predominance and the maintenance of alpha diversity across all expectant mothers.
50% 50% of healthy pregnant women have a vaginal microbiota dominated by L. iners
57% 57% of pregnant women with gestational diabetes have a vaginal microbiota dominated by L. crispatus
Most notably, the type dominated by L. iners was less common in the group of sick women (31.15%) than in the group of healthy women (50%). In addition, L. iners was relatively more abundant (as a percentage of the species present) in healthy women.
Conversely, the type dominated by the protective L. crispatus was paradoxically more frequent among women with gestational diabetes or maternal complications. This finding challenges the conventional understanding of vaginal dysbiosis, which is often oversimplified as merely an imbalance between “good” and “bad” bacteria.
Altered metabolic pathways
The increased abundance of L. iners in healthy pregnant women was associated with the overexpression of metabolic pathways favorable to a healthy pregnancy—for example,
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Tetrahydrofolate
a coenzyme derived from folic acid, primarily involved in the synthesis of nucleic acid bases (purines and pyrimidines), which constitute the DNA and RNA of genetic material. Tetrahydrofolate (THF) is also involved in the synthesis of amino acids, including methionine, histidine, and serine.
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biosynthesis, which may play various roles (microbial folate synthesis, a slightly pro-inflammatory state).
But the higher abundance of this bacterium in healthy women was also associated with
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Glycosyltransferase
a membrane-associated enzyme that catalyzes the transfer of a sugar moiety onto a protein, resulting in the formation of a glycoprotein. In pathogenic bacteria, these glycoproteins have been implicated at various stages of the infection process.
Explore: Tomás JM, Fulton KM, Twine SM et al. Generation of Null Mutants to Elucidate the Role of Bacterial Glycosyltransferases in Bacterial Motility. J Vis Exp. 2022 Mar 11;(181).
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synthesis routes and antibiotic resistance, compared with women suffering from pathologies. These mechanisms may reflect the dynamic adaptation of the microbiota to the immune and hormonal environment characteristic of the third trimester of pregnancy.
Access clear visual aids to explain the role of vaginal microbiota, including the infographic “Women’s microbiota: the missing piece in intimate health”:
Seven very different strains of L. iners
More in-depth analyses show that not all L. iners are alike. Among the seven strains of L. iners identified by the authors, three strains associated with bacterial vaginosis (versus four strains associated with good health) proved to be more efficient at forming biofilms, thanks to genes coding for the proteins involved. The team also showed that five of the seven identified strains (whether associated with bacterial vaginosis or not) inhibited the growth of the pathogen G. vaginalis, which is implicated in preterm birth.
These results suggest that L. iners may exert a protective influence depending on environmental conditions and the strains involved. A microbiota dominated by certain strains of L. iners could therefore contribute to the prevention of complications linked to persistent dysbiosis.
According to the authors, this heterogeneity—particularly in relation to G. vaginalis—warrants further investigation.
This is all the more relevant as L. iners, likely through its metabolic flexibility, appeared to support the stability of the vaginal ecosystem in the study population.
