Abstract
Lactobacillus iners, first described in 1999, is a prevalent bacterial species of the vaginal microbiome. As L. iners does not easily grow on de Man-Rogosa-Sharpe agar, but can grow anaerobically on blood agar, it has been initially overlooked by traditional culture methods. It was not until the wide application of molecular biology techniques that the function of L. iners in the vaginal microbiome was carefully explored. L. iners has the smallest genome among known Lactobacilli and it has many probiotic characteristics, but is partly different from other major vaginal Lactobacillus species, such as L. crispatus, in contributing to the maintenance of a healthy vaginal microbiome. It is not only commonly present in the healthy vagina but quite often recovered in high numbers in bacterial vaginosis (BV). Increasing evidence suggests that L. iners is a transitional species that colonizes after the vaginal environment is disturbed and offers overall less protection against vaginal dysbiosis and, subsequently, leads to BV, sexually transmitted infections, and adverse pregnancy outcomes. Accordingly, under certain conditions, L. iners is a genuine vaginal symbiont, but it also seems to be an opportunistic pathogen. Further studies are necessary to identify the exact role of this intriguing species in vaginal health and diseases.
Highlights
The vaginal microbiome plays an important role in determining human vaginal health
Many studies have reported that the presence of L. crispatus in the vagina is associated with good health, whereas communities dominated by L. iners fail to provide sufficient protection against vaginal dysbiosis (Petricevic et al, 2014; France et al, 2016; Tortelli et al, 2020)
L. iners is usually the only vaginal Lactobacillus species coexisting with bacterial vaginosis (BV)-associated bacteria that can be detected during BV (Macklaim et al, 2011; Santiago et al, 2012)
Summary
The vaginal microbiome plays an important role in determining human vaginal health. Using highthroughput metagenomic and 16S rRNA sequencing, over 250 bacterial species have been identified in the human vagina (Fredricks et al, 2005; Chen et al, 2020). Many studies have reported that the presence of L. crispatus in the vagina is associated with good health, whereas communities dominated by L. iners fail to provide sufficient protection against vaginal dysbiosis (Petricevic et al, 2014; France et al, 2016; Tortelli et al, 2020). L. iners is usually the only vaginal Lactobacillus species coexisting with BV-associated bacteria that can be detected during BV (Macklaim et al, 2011; Santiago et al, 2012) It can persist under the drastically changing vaginal environment of BV due to its ability to respond and regulate its genomic functions (Macklaim et al, 2013). As dynamic changes in the vaginal econiche were characterized at different time points in the menstruation cycle within the same individual, the moment of sampling relative to the menstrual cycle is very important for vaginal community analysis
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