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Lactobacillus-dominated vaginal flora: what are Community State Types (CSTs) and why stability matters

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“LACTOBACILLUS-DOMINATED FLORA”: A COMMON BUT INCREASINGLY IMPRECISE TERM

A “balanced” vaginal microbiota is often described as being characterized by lactobacillus-dominated flora. Although this expression remains widely used, it is now considered scientifically imprecise and only partially informative.

Referring generically to “lactobacilli” does not distinguish between different species, which may perform markedly different biological and ecological roles within the vaginal environment. In other words, the mere presence of lactobacilli is not sufficient to describe the functional state of the vaginal microbiota.

Over the past fifteen years, research has clarified that not all lactobacillus-dominated profiles are equivalent. A more precise and functionally oriented classification system is necessary to fully understand vaginal microbiota physiology.

INDIGENOUS VAGINAL LACTOBACILLI: FEW AND HIGHLY SPECIFIC

The genus Lactobacillus includes numerous species, many of which are widely studied in food science and probiotic applications. However, only a limited number are typically indigenous to the human vaginal ecosystem.

Scientific literature consistently identifies four predominant species:

  • Lactobacillus crispatus
  • Lactobacillus gasseri
  • Lactobacillus iners
  • Lactobacillus jensenii

Notably, well-known species such as Lactobacillus acidophilus, L. plantarum, or L. rhamnosus, although frequently used in probiotic formulations, have not been identified as dominant members of the vaginal microbiota. As such, they appear less adapted to long-term colonization of the vaginal environment and to contributing durably to its ecological balance.

THE SCIENCE OF THE VAGINAL MICROBIOTA: WHAT ARE COMMUNITY STATE TYPES (CSTS)?

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The vaginal microbial community is neither random nor infinitely variable among individuals. Landmark studies published over the past fifteen years — including the pioneering 2011 work by Ravel and colleagues — identified recurring and recognizable compositional patterns of the vaginal microbiota, termed Community State Types (CSTs), sometimes referred to as “vaginotypes.”

CSTs represent dominant compositional structures of the vaginal microbiota, classified according to the prevailing species and the overall microbial community configuration.

Five principal CSTs have been described:

  • CST I: dominated by Lactobacillus crispatus
  • CST II: dominated by Lactobacillus gasseri
  • CST III: dominated by Lactobacillus iners
  • CST V: dominated by Lactobacillus jensenii

These are accompanied by CST IV, characterized by a near-complete depletion of lactobacilli and the presence of a heterogeneous, polymicrobial community of anaerobic bacteria, including Gardnerella, Atopobium, Prevotella, Megasphaera, Sneathia, and Mobiluncus. This configuration reflects a less ecologically stable vaginal microbiota and is frequently associated with conditions of vaginal dysbiosis, a topic explored in greater depth in our dedicated article on bacterial vaginosis, where its characteristics, clinical signs, and management approaches are discussed.

These microorganisms, generally considered opportunistic, are involved in the most common conditions of microbiota imbalance and tend to emerge when lactobacilli lose their capacity to maintain a selective environment characterized by relatively low biodiversity.

In contexts of marked bacterial microbiota disorganization, increased permissiveness to the growth of non-bacterial microorganisms such as Candida may also be observed. Although Candida species are normally present within the vaginal ecosystem, they may proliferate opportunistically when the ecological control exerted by dominant lactobacilli is reduced, as described in our in-depth article on vaginal candidiasis and the role of L. crispatus in microbiota balance.

In some classifications, CST IV is further subdivided into subtypes (IV-A and IV-B), with the latter more frequently associated with more pronounced microbiota disorganization.

WHAT “DOMINANCE” REALLY MEANS IN THE VAGINAL MICROBIOTA

Within the CST framework, “dominance” does not simply indicate presence. It refers to quantitative and functional prevalence.

Dominance is typically defined when a single species constitutes a large majority — often 70–90% or more — of the total microbial community. This predominance enables the dominant species to exert substantial influence on:

  • vaginal pH
  • ecological competition
  • overall microbiota stability

TWO CONSISTENT OBSERVATIONS FROM THE SCIENTIFIC LITERATURE

Beyond classification systems, two observations consistently emerge from the literature:

  • Relatively low vaginal biodiversity is characteristic of more ordered and functionally stable microbiota configurations.
  • Lactobacillus crispatus is the only lactobacillus species consistently associated with eubiotic profiles across different life stages, particularly during reproductive age.

These findings have reshaped the concept of vaginal “balance,” shifting attention from the mere presence of lactobacilli to the quality, structure, and stability of the microbial community.

WHY LACTOBACILLUS CRISPATUS IS CONSIDERED THE MOST EUBIOTIC PROFILE

The central role attributed to Lactobacillus crispatus is supported by a substantial body of scientific literature describing its functional characteristics.

Frequently reported mechanisms include:

  • Contribution to the maintenance of an acidic vaginal pH, creating an environment less favorable to the overgrowth of undesirable microorganisms
  • Strong ecological competitiveness, limiting the expansion of opportunistic bacteria commonly involved in vaginal infections
  • A tendency to promote a more ordered microbial structure, less prone to dysbiotic shifts

Taken together, these features explain why L. crispatus dominance is frequently associated with a microbiota considered more physiologically coherent and ecologically stable.

STABILITY: THE KEY DISCRIMINATING FACTOR AMONG CSTS

An increasingly central aspect in vaginal microbiota research is long-term stability. Not all lactobacillus-dominated CSTs demonstrate the same capacity to remain stable under physiological conditions.

Longitudinal studies, including the Women’s Health and the Vaginal Microbiome (W4H) project, have shown that:

  • Lactobacillus crispatus-dominated profiles (CST I) tend to be particularly stable over time.
  • Other lactobacillus-dominated configurations, especially those dominated by iners (CST III), appear more susceptible to ecological transitions and shifts toward less stable states.

In this context, while L. iners may be compatible with health, L. crispatus is widely regarded as the reference profile for ecological stability of the vaginal microbiota.

BEYOND THE SPECIES: THE IMPORTANCE OF THE STRAIN

Recent research highlights that not all strains within the same species are functionally equivalent. Genetic and metabolic differences between strains may translate into distinct biological behaviors.

For this reason, a growing body of literature focuses on specific strains of Lactobacillus crispatus, including strain M247, which has been investigated in microbiological and clinical-experimental contexts related to the vaginal microbiota.

This strain-level approach reflects an increasingly refined understanding of vaginal ecology — one that considers not only species identity but also the functional quality of its presence.