Vaginal microbiome, Lactobacillus, Bacterial vaginosis

Lactobacillus Explained: The Beneficial Bacteria That Help Protect Vaginal Health

When we talk about the vaginal microbiome, one group of bacteria comes up again and again: Lactobacillus.

In many reproductive-aged women, Lactobacillus species dominate the vaginal microbial community. Unlike the gut microbiome, where high bacterial diversity is often considered beneficial, a healthy vaginal microbiome is frequently characterised by relatively low microbial diversity and dominance by one or a small number of Lactobacillus species.[1]

But there is an important distinction.

Not all Lactobacillus species behave in the same way.

A vaginal microbiome dominated by Lactobacillus crispatus may function quite differently from one dominated by Lactobacillus iners. And having Lactobacillus present does not automatically mean that symptoms cannot occur.

Understanding these differences is becoming increasingly important in women with recurrent bacterial vaginosis (BV), recurrent vaginal symptoms, changing vaginal pH, symptoms around sex or menstruation, and hormonal changes during perimenopause and menopause.

So rather than simply asking:

“Do I have Lactobacillus?”

the more useful questions may be:

Which Lactobacillus species are present? How dominant are they? How stable is the microbial community? What else is present? And how does that information fit with symptoms and established clinical testing?

What Is Lactobacillus?

Lactobacillus refers to a group of lactic-acid-producing bacteria that commonly inhabit the female genital tract.

Landmark vaginal microbiome research by Ravel and colleagues identified several common vaginal microbial community patterns in reproductive-aged women.[1]

Four were predominantly associated with different Lactobacillus species:

• CST I — Lactobacillus crispatus

• CST II — Lactobacillus gasseri

• CST III — Lactobacillus iners

• CST V — Lactobacillus jensenii

Another broad category, CST IV, contained relatively fewer Lactobacillus and a greater abundance and diversity of other bacteria.[1]

These findings changed how we think about vaginal health.

There isn't one universal “normal vaginal microbiome”.

Instead, there are different microbial ecosystems — and those ecosystems can also change over time.

Why Is the Vaginal Microbiome Different From the Gut?

This is one of the most important concepts in vaginal microbiome medicine.

When people hear the word microbiome, they often assume:

More bacterial diversity = healthier microbiome.

That concept is frequently applied to the gut.

It does not translate neatly to the vagina.

During the reproductive years, many vaginal microbial communities associated with health have relatively low diversity and are dominated by Lactobacillus.[1]

These bacteria help create an acidic vaginal environment through the production of lactic acid.[2]

That acidic environment is one of the important ecological features that distinguishes the vagina from many other microbial environments in the body.

How Does Lactobacillus Help Protect the Vaginal Environment?

Lactobacillus does considerably more than simply occupy space.

Its protective effects appear to involve several interconnected mechanisms.

1. Lactic Acid Production

One of the most important functions of vaginal Lactobacillus is the production of lactic acid.

Lactic acid contributes to the characteristically acidic vaginal environment seen in many reproductive-aged women.

A vaginal pH of approximately 3.8–4.5 is commonly associated with Lactobacillus-dominant communities, although pH varies with age, hormones, menstruation, semen exposure and other factors.

Lactic acid also appears to have biological effects beyond simply lowering pH, including effects on microorganisms and host immune responses.[2]

This is one reason vaginal pH can provide useful ecological information — although pH alone cannot diagnose a vaginal condition.

2. Competition With Other Microorganisms

Lactobacilli occupy ecological niches along the vaginal epithelium.

By establishing themselves within this environment, they can influence the ability of other organisms to colonise and expand.

When the ecosystem changes and Lactobacillus dominance decreases, BV-associated anaerobic organisms such as Gardnerella, Prevotella, Fannyhessea and others may become more abundant.

However, this relationship is not simply:

Lactobacillus absent = disease.

Microbial composition needs to be interpreted alongside symptoms and clinical findings.

3. Interaction With the Vaginal Epithelium

The vaginal microbiome and vaginal epithelium continuously interact.

Lactobacillus metabolites can influence epithelial integrity, inflammatory signalling and local immune responses.[2]

This is why I prefer to think of the vaginal microbiome as an ecosystem, rather than simply a list of microorganisms.

Hormones, epithelial cells, mucus, glycogen, immune responses, microbial metabolites and microorganisms all interact.

Lactic Acid: More Than Just Vaginal pH

Lactic acid is particularly interesting because different Lactobacillus species produce different forms of it.

There are two major stereoisomers:

D-lactic acid

and

L-lactic acid.

Some vaginal Lactobacillus species can produce both.

Lactobacillus iners, however, appears to produce predominantly L-lactic acid and lacks some of the metabolic characteristics found in species such as L. crispatus.[3]

These differences may partly explain why different Lactobacillus-dominant communities behave differently.

But an important distinction is needed.

Although laboratory research has identified biological differences between D- and L-lactic acid, we should not interpret D-lactic acid as a clinically established treatment or BV biofilm disruptor.

The science is much more complex than that.

What About Hydrogen Peroxide?

For many years, hydrogen peroxide — H₂O₂ — production was proposed as one of the major reasons vaginal Lactobacillus protects against infection.

You may still see vaginal probiotics marketed on the basis of containing “hydrogen-peroxide-producing Lactobacillus”.

However, contemporary research has questioned how important this mechanism actually is inside the vagina.

Hydrogen peroxide can inhibit microorganisms under certain laboratory conditions, but vaginal conditions differ substantially from those experimental environments.

Current understanding places considerably more emphasis on:

lactic acid, acidic pH, microbial competition, bacterial metabolites and host–microbe interactions

rather than assuming hydrogen peroxide production is the primary protective mechanism.[2]

This is a good example of why vaginal microbiome medicine needs to evolve as the evidence evolves.

Not All Lactobacillus Species Are the Same

This is perhaps the most clinically interesting part of the science.

Two women can both have vaginal microbiomes reported as:

“Lactobacillus dominant”

while having quite different microbial ecosystems.

Lactobacillus crispatus

Lactobacillus crispatus is one of the most extensively studied vaginal Lactobacillus species.

Crispatus-dominant communities are frequently associated with:

• lower vaginal pH

• lower microbial diversity

• greater microbial stability

• lower prevalence of BV-associated communities

• favourable genital-health outcomes in a number of observational studies.[1,4]

For this reason, CST I — the L. crispatus-dominant community — is often regarded as one of the more stable Lactobacillus-dominant vaginal states.

But even here, we need nuance.

Finding L. crispatus does not guarantee absence of disease, and simply detecting it on a molecular test does not tell us everything about vaginal function.

Lactobacillus iners: The Fascinating One

Lactobacillus iners deserves its own discussion.

It is extremely common.

It can dominate the vaginal microbiome of women without symptoms.

It can also persist during BV-associated changes and appear rapidly after antimicrobial treatment.[3]

This has led researchers to describe L. iners as an unusually adaptable vaginal species.

Its genome is substantially smaller than those of several other vaginal Lactobacillus species, and its metabolic capabilities differ from L. crispatus, L. gasseri and L. jensenii.[3]

Importantly:

Lactobacillus iners is not a pathogen.

Finding L. iners does not mean a woman has BV or vaginal disease.

However, L. iners-dominant communities appear, on average, to behave differently from L. crispatus-dominant communities.

Systematic reviews and longitudinal research suggest L. iners dominance can be associated with less microbial stability and a greater likelihood of transition toward BV-associated community states compared with L. crispatus dominance.[3–5]

This distinction matters enormously when interpreting comprehensive vaginal microbiome results.

Lactobacillus gasseri and Lactobacillus jensenii

These are two other commonly recognised vaginal Lactobacillus species.

Both can dominate healthy vaginal communities and are represented in the original CST classification:

CST II — L. gasseri
CST V — L. jensenii.[1]

They are studied less extensively than L. crispatus and L. iners, but both contribute to vaginal lactic-acid production and can form part of Lactobacillus-dominant vaginal ecosystems.

Again, the important point is:

“Lactobacillus” is not one organism.

Species matter.

And increasingly, research suggests strain-level differences may matter as well.[3]

What Are Vaginal Community State Types?

Community State Types — CSTs — are a research framework used to group vaginal microbial communities according to their predominant organisms.

The classic model described:

CST I: L. crispatus dominant
CST II: L. gasseri dominant
CST III: L. iners dominant
CST IV: lower Lactobacillus with greater abundance/diversity of other bacteria
CST V: L. jensenii dominant.[1]

CSTs have been enormously useful for understanding vaginal microbial ecology.

However, they are not clinical diagnoses.

A CST cannot independently diagnose BV, aerobic vaginitis, candidiasis or another vaginal condition.

Is CST IV Always Vaginal Dysbiosis?

No.

This is another area where overly simplified microbiome information can become misleading.

CST IV generally describes communities with relatively low Lactobacillus abundance and greater bacterial diversity.

BV frequently occurs within these types of microbial communities.

But:

CST IV does not automatically mean BV.

And it does not automatically mean disease.

Some asymptomatic women have low-Lactobacillus vaginal communities.[1]

Clinical interpretation therefore needs to consider:

microbiology + symptoms + pH + established testing + clinical history.

This is particularly important when interpreting commercial vaginal microbiome tests.

The Vaginal Microbiome Is Dynamic

Your vaginal microbiome is not fixed for life.

It can change — sometimes quickly.

Longitudinal research shows vaginal microbial communities can transition between different states over time.[6]

Factors that may influence those transitions include:

• menstruation

• sexual activity

• semen exposure

• antimicrobial use

• hormonal changes

• pregnancy

• menopause

• vaginal products

• host immune and epithelial factors.

In some women, the microbiome returns rapidly to its previous state.

In others, disruption may be followed by a more persistent shift.

This brings us to an increasingly useful concept:

Microbial Resilience

Rather than thinking only about whether a vaginal microbiome is “good” or “bad”, it may be more useful to ask:

How resilient is the ecosystem?

A resilient microbial community can experience a disturbance and subsequently return toward its previous functional state.

This may eventually prove particularly important in understanding why some women experience an isolated episode of BV while others develop repeated recurrence.

However, vaginal microbial resilience remains an evolving research concept rather than a routine clinical measurement.

What Can Cause Lactobacillus Communities to Change?

There is rarely one universal cause.

Different influences can alter the vaginal environment.

Menstruation

Menstruation temporarily changes vaginal chemistry and pH.

Longitudinal research has found menstruation associated with transitions toward low-Lactobacillus molecular BV states in some women.[5]

For many women, Lactobacillus dominance subsequently re-establishes.

For others, symptoms may repeatedly cluster around menstruation.

That pattern can be clinically informative.

Sex and the Vaginal Microbiome

Sex can influence the vaginal environment through several mechanisms.

Semen is alkaline relative to the normally acidic reproductive-aged vagina, so unprotected vaginal intercourse can temporarily raise vaginal pH.

Sex also involves microbial exchange between partners.

This has become especially important in recurrent BV.

Recent evidence has changed our understanding of partner involvement in BV recurrence, and current Australian STI guidance now incorporates concurrent treatment of an ongoing male partner in appropriate BV cases.

This is a significant shift from the older idea that BV should be considered purely an individual vaginal microbiome disorder.

It does not, however, mean that every change in vaginal Lactobacillus after sex represents infection.

Hormones, Oestrogen and Lactobacillus

Oestrogen has an important relationship with the vaginal environment.

During reproductive years, oestrogen supports maturation of the vaginal epithelium and accumulation of glycogen-related substrates that help shape vaginal microbial ecology.

As oestrogen levels decline during perimenopause and menopause, the vaginal epithelium and microbial environment can change.

Studies have found lower Lactobacillus abundance in some peri- and postmenopausal women, particularly in association with vulvovaginal atrophic changes.[7]

This helps explain why menopause can be accompanied by:

• increased vaginal pH

• dryness

• burning

• irritation

• urinary symptoms

• altered vaginal microbial composition.

But menopause-related symptoms should not automatically be diagnosed as infection.

Genitourinary syndrome of menopause — GSM — can produce symptoms that overlap considerably with infection.

Antibiotics and Microbial Recovery

Antibiotics can alter microbial communities.

This does not mean that every antibiotic course permanently “destroys” vaginal Lactobacillus.

Some women rapidly recover their previous microbial state.

Others transition into a different community.

And the effect depends on the antibiotic, the organisms present before treatment, host factors and the underlying condition being treated.

So I prefer the concept:

DISRUPTION → RECOVERY → RESILIENCE

rather than:

ANTIBIOTICS → LACTOBACILLUS DEPLETION.

That distinction matters.

Lactobacillus and Bacterial Vaginosis

BV represents one of the clearest examples of major vaginal microbial ecosystem change.

BV is generally characterised by a reduction in Lactobacillus dominance alongside increased abundance of multiple anaerobic organisms.

These may include:

Gardnerella
Fannyhessea vaginae
Prevotella
Mobiluncus
and other BV-associated bacteria.

Biofilm formation — particularly involving Gardnerella and other organisms — adds another layer of complexity.

This may help explain why BV can recur even after apparently successful antimicrobial treatment.

Importantly, treating BV and establishing a stable Lactobacillus-dominant ecosystem are not necessarily the same biological event.

Can Lactobacillus Help Prevent Recurrent BV?

This is one of the most exciting areas of vaginal microbiome research.

The best-known study is the LACTIN-V trial.[8]

LACTIN-V contains a specific strain:

Lactobacillus crispatus CTV-05.

Women first received treatment for BV with vaginal metronidazole and were then randomised to receive vaginal LACTIN-V or placebo.

By 12 weeks:

30% of women receiving LACTIN-V had recurrent BV

compared with

45% receiving placebo.[8]

That is clinically interesting.

But it does not mean:

Any probiotic containing L. crispatus will produce the same result.

The evidence applies to the specific strain, formulation, dose, route and protocol studied.

A later analysis also suggested the preventative effect was most apparent in women who had achieved clinical BV cure following initial antibiotic treatment.[9]

That supports an important ecological concept:

First treat the active disorder. Then investigate strategies aimed at establishing a more stable vaginal ecosystem.

Species Is Not the Same as Strain

This deserves emphasis.

When choosing or evaluating a probiotic, there are several levels:

Genus → Species → Strain → Formulation → Dose → Route

For example:

Lactobacillus = genus

Lactobacillus crispatus = species

Lactobacillus crispatus CTV-05 = specific strain studied in LACTIN-V.

We cannot automatically transfer evidence from one strain to every commercial product containing the same species.

This is one of the biggest problems with generic claims about “vaginal probiotics”.

What About Recurrent Thrush?

This is another area where microbiome information is frequently oversimplified.

Recurrent vulvovaginal candidiasis should not simply be viewed as a disorder caused by insufficient Lactobacillus.

Candida-associated disease can occur in women with Lactobacillus-dominant vaginal communities.

Symptoms such as:

itching, burning, irritation and white discharge

therefore cannot tell us whether a woman has:

• candidiasis

• BV

• cytolytic vaginosis

• aerobic vaginitis

• dermatitis

• GSM

• another vulvovaginal condition.

Repeatedly treating symptoms as “thrush” without confirming the diagnosis can therefore miss the underlying problem.

Can You Have Too Much Lactobacillus?

Potentially — but this is an area where we need to be scientifically careful.

A proposed condition known as cytolytic vaginosis describes women with thrush-like symptoms associated with:

• abundant lactobacilli

• epithelial cytolysis

• acidic vaginal pH

• absence of Candida.

The concept has existed for decades, but diagnostic criteria and pathophysiology remain less firmly established than conditions such as BV or vulvovaginal candidiasis.

It nevertheless illustrates an important principle:

More Lactobacillus does not automatically equal fewer vaginal symptoms.

Symptoms still require diagnosis.

Lactobacillus and Recurrent UTI

The vagina and urinary tract should not necessarily be considered completely separate microbial environments.

The periurethral and vaginal environments can influence exposure of the urinary tract to uropathogens.

This relationship becomes particularly interesting around menopause, when declining oestrogen may coincide with changes in vaginal Lactobacillus populations and increased susceptibility to recurrent UTI.

However, vaginal microbiome testing is not currently a standard diagnostic test for recurrent UTI.

The relationship is biologically plausible and increasingly studied, but treatment should remain grounded in established UTI assessment and prevention strategies.

What Can Vaginal Microbiome Testing Tell Us?

Comprehensive molecular testing can provide information conventional vaginal testing may not show.

Depending on the assay, it may identify:

• dominant Lactobacillus species

• relative Lactobacillus abundance

• BV-associated anaerobes

• aerobic organisms

• Candida species

• other detected microorganisms.

This can be particularly interesting in women experiencing recurrent or unexplained symptoms.

But there is a critical principle:

Detection does not equal disease.

Finding an organism does not prove that organism is causing the patient's symptoms.

Likewise, relative abundance is not the same as pathogenicity.

Microbiome testing therefore needs to be interpreted alongside:

symptoms, clinical history, vaginal pH where appropriate, established pathology testing, treatment history, hormonal status and recurrence patterns.

There Is No Universal “Healthy Lactobacillus Number”

Some commercial microbiome tests report absolute bacterial counts.

Others report relative abundance.

Others use proprietary reference ranges.

There is currently no single universal Lactobacillus concentration that defines a healthy vagina across every testing platform.

That is why I would not interpret a result such as:

“Lactobacillus below X CFU/mL”

in isolation.

Laboratory methodology matters.

Species matter.

Relative abundance matters.

And most importantly:

the patient matters.

How I Think About Lactobacillus in Recurrent Vaginal Symptoms

When I assess a woman with recurrent symptoms, I don't simply ask whether Lactobacillus is present.

I am looking at the whole ecological and clinical picture:

What Lactobacillus species dominate?

Is the community Lactobacillus dominant or depleted?

Is L. crispatus present?

Is L. iners dominant?

Are BV-associated organisms present?

Is Candida present — and which species?

What is happening with vaginal pH?

Do symptoms occur after sex?

Do they occur after menstruation?

Has there been repeated antimicrobial exposure?

Is the patient perimenopausal or postmenopausal?

Are urinary symptoms occurring as well?

Does the microbiology actually match the symptoms?

This is why recurrent vaginal symptoms often need a broader approach than simply repeating another course of the treatment that worked temporarily last time.

An Evidence-Informed Framework

In my clinic, I organise complex recurrent vaginal microbiome cases using an evidence-informed framework:

RESET → REBUILD → RESTORE → MAINTAIN → RESCUE

Importantly, this is my clinical framework for organising assessment and management, not a published guideline or universally validated treatment protocol.

RESET

Identify and appropriately treat the condition actually present.

That might be BV, candidiasis, aerobic vaginitis, an STI, GSM or another condition.

REBUILD

Consider what factors may support recovery of the vaginal environment after the active disorder has been addressed.

RESTORE

Where clinically appropriate, the aim may be to support a stable vaginal environment rather than simply achieving temporary symptom suppression.

MAINTAIN

Identify recurrence drivers.

These might include sexual exposure, hormonal change, recurrent antimicrobial use, menstrual patterns or other individual factors.

RESCUE

Have a clinically appropriate plan for recurrence rather than automatically repeating empirical treatment indefinitely.

The specific management strategy is individualised because BV, recurrent candidiasis, aerobic vaginitis, GSM and recurrent UTI are biologically different conditions.

The Bigger Picture

The science of Lactobacillus has moved far beyond:

“Good bacteria protect the vagina.”

We now understand that vaginal microbial ecology is much more sophisticated.

Different Lactobacillus species have different genomes, metabolites and ecological behaviours.

Crispatus and iners are not interchangeable.

A Lactobacillus-dominant microbiome can change.

A low-Lactobacillus microbiome does not automatically mean disease.

CSTs are useful research tools but are not diagnoses.

And microbiome testing can provide valuable additional information — but it must be interpreted clinically.

Perhaps the most important concept is this:

Vaginal health is not simply about the presence or absence of one bacterium.

It is about the relationship between the microbiome, vaginal epithelium, hormones, immune system, pH, sexual exposures, microbial metabolites and the individual woman's symptoms.

That is why recurrent vaginal symptoms deserve more than simply asking:

“Which infection do we treat next?”

Sometimes the more useful question is:

“What is happening to the vaginal ecosystem — and why does it keep changing?”