Introduction In case of bacterial vaginosis (BV), lactobacilli are outnumbered by anaerobic bacteria. These bacteria have been described to form a polymicrobial biofilm (“clue cells”) attached to the vaginal epithelium, possibly explaining the decreased sensitivity to antibiotic treatment and the frequent chronicity of BV. A better understanding of this biofilm may contribute to more precise delineation, and consequently to better diagnosis and treatment of BV. We used Fluorescence In Situ Hybridization (FISH) to study the presence of Atopobium vaginae (Av) and Gardnerella vaginalis (Gv) biofilm, in relation to the status of the vaginal microbiome (according to Nugent score).

Methods Duplicates of 461 vaginal slides of 120 participants, participating in a contraceptive vaginal ring trial in Rwanda, were evaluated according to Nugent score after Gram staining and by FISH with species-specific probes for Av and Gv.

Results In the majority of samples (59.6%) with Nugent score 7–10 (BV diagnosis), a Gv+Av biofilm was visualised by FISH. In these BV samples, 7% had no Gv and Av present, in 9% Gv and/or Av was present but only planktonic, 18.6% had Gv biofilm but no Av present and 5.8% Gv biofilm with Av planktonic only. When Av and/or Gv were planktonic in the vagina and not part of a biofilm, the Nugent score was low, defining a healthy vaginal microbiome. However the probability of having a disturbed vaginal microbiome (Nugent 7–10) was increased (p < 0.001) when a biofilm was visualised with FISH. Moreover, the probability for having a Nugent score of 7–10 was increasing when A. vaginae was part of the biofilm (p < 0.001).

Conclusion Our study focusing on Gv and Av, shows that these are two major players in a polymicrobial condition. It confirms the importance of Gv-biofilm in BV and strongly indicates that Av plays an important role in BV-associated biofilm.

Disclosure of interest statement This work was supported by European and Developing Countries Clinical Trials Partnerships (EDCTP), by Combined Highly Active Anti-Retroviral Microbicides (CHAARM) and by Dormeur Investment Service. No pharmaceutical grants were received in the development of this study.