Lactobacillus iners

Metal Dependencies#

Manganese: L. iners relies on manganese-dependent enzymes for Oxidative Stress defense. Unlike L. crispatus, which produces robust hydrogen peroxide, L. iners produces minimal H2O2—a critical functional deficit.

Iron: Required for basic metabolic processes, though L. iners has a streamlined genome with reduced biosynthetic capacity compared to other vaginal lactobacilli.

Key Enzymes and Virulence Factors#

The key distinction between L. iners and protective vaginal lactobacilli lies in its enzymatic profile. Inerolysin: A cholesterol-dependent cytolysin (CDC) unique to L. iners. This pore-forming toxin can damage vaginal epithelial cells and may contribute to the breakdown of the mucosal barrier.

No other vaginal Lactobacillus produces a CDC.

L-lactate dehydrogenase: L. iners produces only L-lactic acid, whereas L. crispatus produces both D- and L-lactic acid. D-lactic acid is the form with stronger antimicrobial activity and is associated with greater protection against HIV and other sexually transmitted infections.

Absent H2O2 production: L. iners produces little to no hydrogen peroxide, eliminating a key antimicrobial mechanism that other vaginal lactobacilli use to suppress pathobionts.

Minimal bacteriocin production: Unlike L. crispatus, L. iners has a reduced genome (~1.3 Mb, among the smallest of lactobacilli) with limited capacity for antimicrobial peptide synthesis.

Ecological Role#

L. iners occupies a paradoxical ecological position—it is a Lactobacillus that does not protect. Transitional state indicator: L. iners-dominant communities are frequently the transitional state between L. crispatus dominance (healthy) and BV-associated polymicrobial dysbiosis. Women can shift rapidly from CST-III to CST-IV (BV) and back.

Pathogen permissive: In women with tubal infertility and Chlamydia trachomatis infection, the vaginal microbiota was L. iners-dominated rather than L. crispatus-dominated, with significantly lower Shannon diversity. L. crispatus (not L. iners) is the species that restricts CT colonization (,^{[1]Chen 2021 — Alterations of vaginal microbiota in women with infertility and Chlamydia trachomatis infectionChen H, Wang L, Zhao L et al. · 2021Open reference 1 ↓} case-control, n=25).

Failure to suppress S. aureus: In PCOS vaginal microbiomes, L. iners did not correlate with S. aureus suppression, reinforcing that it lacks the antimicrobial activity of L. crispatus (,^{[2]Zheng 2024 — Differential enrichment of bacteria and phages in vaginal microbiomes in PCOS and obesity: shotgun sequencing analysisZheng S, Chen H, Yang H et al. · 2024Open reference 2 ↓} cross-sectional).

Enriched in:#

Endometriosis: L. iners significantly reduced in endometriosis-associated chronic pelvic pain patients alongside L. jensenii and L. reuteri, while pathogenic Clostridia expanded (,^{[3]The role of the vaginal microbiome in distinguishing female chronic pelvic pain caused by endometriosis/adenomyosisChao X, Liu Y, Fan Q et al. · 2021Open reference 3 ↓} cross-sectional). Separately, L. iners was implicated in endometriosis vaginal microbiota-N-glycome studies (,^{[4]MacSharry 2024 — Endometriosis Specific Vaginal Microbiota Links to Urine and Serum N-GlycomeJohn MacSharry, Zsuzsanna Kovacs, Yongjing Xie et al. · 2024Open reference 4 ↓} case-control).

Ovarian cancer (serous histology): L. iners enriched in the lower reproductive tract of serous ovarian cancer patients alongside Facklamia hominis, Anaerococcus senegalensis, and Actinomyces turicensis. Consistent with L. iners as a dysbiosis marker rather than a eubiosis marker (,^{[5]Asangba 2023 — Diagnostic and prognostic potential of the microbiome in ovarian cancer treatment responseAsangba AE, Chen J, Goergen KM et al. · 2023Open reference 5 ↓} cross-sectional).

PCOS: Part of the altered vaginal microbiome in PCOS patients, where shotgun sequencing confirmed dysbiosis with reduced Lactobacillus diversity. Pathobionts including Streptococcus pyogenes and Clostridium perfringens were enriched in PCOS groups (,^{[2]Zheng 2024 — Differential enrichment of bacteria and phages in vaginal microbiomes in PCOS and obesity: shotgun sequencing analysisZheng S, Chen H, Yang H et al. · 2024Open reference 2 ↓} cross-sectional).

Tubal infertility: L. iners-dominated vaginal microbiota was associated with C. trachomatis infection and significantly lower diversity compared to healthy controls (,^{[1]Chen 2021 — Alterations of vaginal microbiota in women with infertility and Chlamydia trachomatis infectionChen H, Wang L, Zhao L et al. · 2021Open reference 1 ↓} case-control, n=25).