Scientific Rationale

From Penile Microbiome to Maternal Outcomes

This page presents the causal pathway that grounds Circunaro's research hypothesis, the evidence supporting each step, and the gap our intervention aims to fill.

The Hypothesis

A Five-Step Causal Pathway

Male Reservoir

Anaerobic bacteria colonize the subpreputial space of uncircumcised men

Sexual Transmission

These bacteria are transferred to female partners during intercourse

Vaginal Dysbiosis

Transmitted anaerobes disrupt the Lactobacillus-dominant vaginal microbiome (BV)

Ascending Infection

Dysbiosis triggers cervical inflammation and ascending intrauterine infection

Adverse Outcomes

Preterm birth, low birth weight, neonatal sepsis

This five-step causal chain is our working model, not an established fact. The early links are supported by observational evidence and one randomized trial (StepUp, with antibiotics). Whether the male reservoir causes recurrence, and whether removing it without antibiotics reduces recurrence, are hypotheses. Pregnancy outcomes have not been studied in partner-directed interventions.

The Evidence Base

Key Studies

RCT · NEJM 2025

StepUp Trial (Vodstrcil et al., 2025)

An open-label randomized trial in monogamous couples in Australia. Treating the male partner with oral metronidazole plus topical 2% clindamycin cream for 7 days reduced BV recurrence in the woman within 12 weeks from 63% (43/68) to 35% (24/69); hazard ratio 0.37 (95% CI 0.22–0.61). The trial was stopped early for benefit. It was open-label, followed women for 12 weeks only, and tested antibiotics.

Why it matters: Shows that treating the male partner can reduce BV recurrence. It does not show that a non-antibiotic approach works, and it did not measure pregnancy outcomes.

63% → 35% BV recurrence at 12 weeks: control arm → partner-treated arm (antibiotics)

View study (DOI) →

RCT · Rakai, Uganda

Rakai Circumcision Trial (Gray et al., 2009)

In a randomized trial in Rakai, Uganda, female partners of circumcised men had a lower prevalence of BV one year later than partners of uncircumcised men (adjusted prevalence ratio 0.60 for any BV; 0.39 for severe BV). Prevalence ratios are not risk reductions.

Why it matters: Supports an association between the male partner's foreskin status and the woman's BV. It is a secondary finding of an HIV prevention trial.

0.60 Adjusted prevalence ratio of any BV in partners of circumcised vs. uncircumcised men, 1 year

Prospective Cohort · Kisumu, Kenya

Penile Microbiome and Incident BV (Mehta et al., 2020)

In a cohort of couples in Kisumu, Kenya, a cross-validated model using bacterial taxa at the male partner's penile meatus discriminated which women developed BV, with an AUC of about 0.89. This is a prediction, not proof that the bacteria were transferred or caused BV.

Why it matters: Shows that the male microbiome carries information about the partner's BV. Cause and direction of transfer are not established.

0.89 Discrimination (AUC) of a model using male penile taxa to predict partner BV · Kisumu, Kenya

View study (DOI) →

Multiple Cohorts

Vaginal Microbiome and Preterm Birth

Several cohort studies associate a vaginal microbiome with few Lactobacillus and high anaerobic diversity with a higher risk of spontaneous preterm birth. Whether treating the male partner changes that risk has not been studied.

Why it matters: Explains why vaginal health matters in pregnancy. Our current work focuses on BV recurrence, not on pregnancy outcomes.

The Gap

The Open Question: Without Antibiotics?

StepUp showed that treating the male partner with antibiotics can reduce BV recurrence. Repeated antibiotic courses are not an attractive long-term strategy, and the other established option, circumcision, is irreversible. We are not aware of a non-antibiotic, non-surgical approach for the male partner that has been tested for this purpose.

Surgical circumcision

Associated with lower BV prevalence in partners in the Rakai trial, but irreversible, culturally contested and dependent on surgical infrastructure.

Antibiotics for the partner

Reduced recurrence in StepUp over 12 weeks. Longer-term effects are unknown, and repeated courses raise antimicrobial resistance concerns.

Treating the woman only

Standard care. In the StepUp control arm, 63% of women had BV again within 12 weeks.

Whether a non-antibiotic, partner-applied approach could achieve a similar effect is the question we are working on. It is a hypothesis that has not been tested.

Honest Assessment

Important Assumptions and Research Gaps

Core assumption: retraction ≈ circumcision

This hypothesis assumes that temporary mechanical retraction of the foreskin will produce similar microbiological effects to surgical circumcision. However, circumcision is permanent and removes tissue, while the ring provides temporary retraction. This assumption requires direct testing and is a central aim of our pilot study.

Confounding factors

Sexual behaviour, hygiene practices, and partner characteristics are all potential confounders in studies of BV transmission. Well-designed trials must control for these variables to isolate the device's effect.

Female partner perspective

The intervention addresses the male side of the transmission pathway. Its acceptability to female partners, and the dynamics of male engagement in reproductive health interventions, require dedicated investigation.

Circunaro's Approach

Mechanical Aeration of the Subpreputial Space

The Circunaro ring holds the foreskin retracted, exposing the glans and coronal sulcus to air. The research hypothesis is that sustained exposure to oxygen may disfavour obligate anaerobes.

Circumcision is associated with lower levels of penile anaerobes and with a lower prevalence of BV in female partners. We hypothesize that the ring could produce a similar ecological shift without surgery. This has not been tested.

This hypothesis requires rigorous clinical validation. Circunaro is seeking research partners to design and conduct the studies needed to test it.

About the Device Explore Collaboration

Note: The Circunaro ring is supplied only for research. It has not been shown to prevent or treat any condition, and every mechanism described here is a hypothesis under investigation.

Current Research Activity

Study in Preparation

ADJUNCT-SYPH — Mechanistic Pilot RCT · Rakai, Uganda

In collaboration with the Africa Medical and Behavioral Sciences Organization (AMBSO) and the Rakai Health Sciences Program (RHSP), we are preparing ADJUNCT-SYPH: a mechanistic pilot RCT in Rakai, Uganda. The pilot will test whether continuous wear of the subpreputial ring over eight weeks shifts the male genital microbiome toward a post-circumcision-like state. If the pilot meets its Stop/Go criteria, findings will directly inform a definitive dyadic RCT — testing a combined package of microbiome modification and integrated partner engagement as an adjunct to BPG, with the aim of reducing syphilis transmission in settings where male partners rarely reach the clinic.

Full protocol and rationale →

The study is in preparation phase, pending ethics committee review and funder decision. No results are yet available.

Bibliography

References

Vodstrcil LA, et al. (2025). Treatment of male partners to prevent recurrence of bacterial vaginosis (StepUp). N Engl J Med. 392:947–957. DOI →
Gray RH, et al. (2009). Male circumcision and the risk of bacterial vaginosis in female partners: Rakai, Uganda. Am J Obstet Gynecol. (Citation to be re-verified against the original paper.)
Mehta SD, et al. (2020). Penile microbiome and incident bacterial vaginosis in female partners, Kisumu, Kenya. Front Cell Infect Microbiol. DOI →

Last reviewed: October 2026. References listed here have been checked against the original publications; others previously listed were removed pending verification.