Lactobacillus iners

Lactobacillus iners origin Lactobacillus iners was first discovered in 1999 as a bacterial species prevalent in the vaginal microbiome. L. iners does not grow easily. However, the bacterium can grow anaerobically on blood agar. The bacterium has been initially overlooked by traditional cultural methods. Due to the wide application of molecular biology techniques, the function L. iners was carefully explored in the vaginal microbiome.[Lactobacillus iners]

Among the known Lactobacilli, it has the smallest genome. The bacterium has many probiotic properties. However, in maintaining a healthy vaginal microbiome, L. iners is partly different from other vaginal Lactobacillus species. It is commonly present in the healthy vagina and is often found highly abundant in bacterial vaginosis (BV). L. iners is a transitional species. It colonizes after the disturbance of the vaginal environment.

L. iners does not provide much protection against vaginal dysbiosis. As a result, it leads to adverse pregnancy outcomes, BV, and sexually transmitted infections. Therefore, under specific conditions, the bacterium is a genuine vaginal symbiont. However, L. iners can be an opportunistic pathogen.[Lactobacillus iners]

Lactobacillus iners;
Microscopic images of Lactobacillus iners

Culture characteristics

L. iners is characterized by circular, small, translucent, smooth, and non-pigmented colonies after anaerobic incubation for 24 h on blood agar. Most L. iners isolates grow on MRS agar after adding 1–5% human blood or sheep blood. L. iners tend to grow on MRS agar in anaerobic conditions for at least 7 days, which is longer when compared with other Lactobacillus species.

In MRS broth containing 0.5% cysteine, which acts as a reducing agent, under anaerobic conditions, L. iners has slow growth and reaches its highest value of 10 7 CFU/ml. Consequently, it has slow growth after 12 h.

Lactobacillus iners;
SEM image of Lactobacillus iners; image source:

L. iners and vaginal health Recent data show that the vaginal econiche is dominated by Lactobacillus species without vaginal dysbiosis in women. A vaginal community type dominated by L. iners is less stable or more in transition and is associated with vaginal dysbiosis when compared with other community types. Genome sequencing showed that L. iners has a small genome leading to a symbiotic or parasitic lifestyle in the human vagina.[Lactobacillus iners]

Functional and biochemical assays show that L. iners contains features of both vaginal pathogens and probiotic lactobacilli. The vaginal microbial community is found to have abundant lactobacilli. Lactobacillus iners, which is a relatively recently detected species, is frequently found in the vaginal niche.

But the role of L. iners in vaginal health is unclear because it can be found both during vaginal dysbiosis and under normal conditions, such as bacterial vaginosis, which is a condition having a lack of typical Lactobacilli and exhibits an abnormal rise in bacterial diversity. When compared with
other Lactobacillus species, L. iners has a Gram-variable morphology and more complex nutritional requirements.

Unusually, it has a small genome (ca. 1 Mbp), indicating a parasitic or symbiotic lifestyle. This is in contrast to other Lactobacilli, which show genomes of up to 3–4 Mbp and niche flexibility. Specific L. iners genes, for example, those encoding unique σ-factors and iron-sulfur proteins, exhibit a niche specification of high degree.[Lactobacillus iners]

The genome of the bacterial strains also encodes inerolysin, which is a pore-forming toxin and is related to vaginolysin of Gardnerella vaginalis. This organism has clonal variants, which in some cases promote a healthy vagina, while in other cases, these are linked to disease and dysbiosis.

Lactobacillus iners and vaginal health;
Schematic of Lactobacillus iners
and vaginal health

L. iners and infertility

Previous studies showed that 40% of patients who did not have success with assisted reproduction by invitro fertilization (IVF) had an abnormal reproductive tract microbiome. Vaginal dysbiosis including increased flora diversity, an elevated pH value, BV, trichomonal vaginitis, and vulvovaginal candidiasis are found to be risk factors for infertility. The high amount of L. iners was linked to an increased infertility rate.[Lactobacillus iners]

It was recently reported that the vaginal microbiome dominated by L. iners was linked to C. trachomatis infection and tubal infertility. L. iners, as a transitional species, can facilitate the transition between a normal and an abnormal vaginal microbiome under treatment or high estrogen levels induced artificially and needed for IVF. It is found that the vaginal microbiome dominated by L. iners is not a favorable factor for pregnancy.

L. iners and menstruation cycle The Human Microbiome Project reported that for all body parts, in the microbial community including the vagina, over time, within-subject variations are consistently lower than between-subject variations. The menstrual cycle is the most important factor that disturbs the diversity of the vaginal microbiome.

Usually, L. crispatus plays a dominant role in the vagina of reproductive-age women; however, during the menstruation cycle, L. iners overgrows and L. crispatus is replaced by L. iners. A recent study showed that L. iners, in the follicular phase, was the most recurrent microbe; in the periovulatory phase, CST IV and L. iners were predominant; and the most frequent type, in the luteal phase, was CST IV. The amount of L. iners significantly increases during menses, often in conjunction with an elevation of Atopobium vaginae and/or G. vaginalis. But L. iners decrease subsequently after menses without any intervention.


In conclusion, L. iners is an intriguing Lactobacillus species and is unique with extraordinary characteristics. Its concurrent nutrient dependency and small genome are conducive such that it is highly adaptable to both high and low pH vaginal environments, as well as in both BV-negative and BV-positive conditions. Therefore, L. iners is frequently categorized as a transitional species, which, after an ecological disturbance, colonizes the vagina. But whether L. iners, for the host’s microbiome, is pathogenic or beneficial remains controversial.

Most researchers believe that L. iners can lead to the onset and maintenance of vaginal dysbiosis and does not provide much protection against vaginal colonization by pathogens. L. iners can also be a risk factor for adverse pregnancy outcomes and sexually transmitted infections. A high understanding of the roles that L. iners plays in diseases and health in individuals of different ethnicities and races is required.

Further studies are needed to clarify the role that L. iners plays in the regulation of vaginal mucosal immunity and to clarify if it may be involved as a novel biomarker to detect the prognosis and existence of vaginal inflammation and guide clinical treatment subsequently.[Lactobacillus iners]

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By Ramya Kasinathan

Date: 30/6/2022

Tags: life science

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