Social interactions between Lactobacillus iners and Gardnerella vaginalis biofilms: an unexpected friendship in the bacterial vaginosis ecosystem

Abstract

Background Worldwide, bacterial vaginosis(BV) is the leading dysbiosis of the vaginal microbiome. BV is a complex polymicrobial condition characterized by a disruption of the vaginal econiche, normally resulting in a reduction of beneficial lactobacilli and an overgrowth of anaerobes. It is noteworthy that a hallmark feature of BV is the presence of a highly structured polymicrobial biofilm, primarily consisting of Gardnerella vaginalis, strongly adhered to the vaginal epithelium, and a variety of other bacteria. There are some observational studies that described the rapid fluctuation over time of the vaginal microflora, showing that Lactobacillus inersis a dominant part of the vaginal flora in a transitional stage between abnormal and normal flora. Compared to other Lactobacillus species, L. iners has more complex nutritional requirements, a Gram- variable morphology and an unusually small genome, indicative of a symbiotic or parasitic lifestyle. However, till date, the role of L. iners in the development of a BV- associated biofilm remains unclear. Method This study aimed to unravel the interactions between G. vaginalis and L. iners, both isolated from BV cases, using an in vitro dual - species biofilm assembly. Bacterial coaggregation ability was determined for single - or between dual - species community. Furthermore, the total biofilm biomass was also determined by the crystal violet method. Next, we discriminated the dual - species populations in the biofilm by using Peptide Nucleic Acid Fluorescence in situ Hybridization method. Additionally, biofilm structure was evaluated using a confocal laser scanning microscopy analysis. Finally, the transcripts levels of G. vaginalis virulence genes, in a dual-species consortium, were determined by quantitative PCR. Results & Conclusions This study pointed out that L. iners seems to be well adapted to BV dysbiosis. We observed that L. iners was able to incorporate a pre-established G. vaginalis biofilm. Confocal microscopy analysis revealed that both species can live in close proximity, forming clusters from the bottom to the biofilm top layer. Curiously, L. iners did not affect G. vaginalis virulence,as determined by the transcription levels of key virulence genes. Remarkably, one could argue that L. iners is capable of surviving and adapting to a metabolic stress - related conditions found in BV