Bacteria rarely exist as single-species planktonic forms but thrive in complex polymicrobial adhering communities enveloped by extracellular matrices, so-called biofilms. The bacteria account for less than 10% of biofilm mass, whereas the biofilm matrix usually accounts for more than 90% and provides the best living conditions for the bacteria [
29]. Costerton et al. in 1999 [
30] have described the association of a bacterial biofilm with various chronic infections. Regardless of the location in the human body, biofilm infections share similar clinical characteristics. They grow slowly, and bacterial communities are rarely fully destroyed by the host-defence mechanisms. Bacteria in biofilm release antigens resulting in an increase in antibody production. However, due to the biofilm structure, the produced antibodies are not capable of killing the biofilm bacteria [
30].
The biofilm formation in BV is a virulence mechanism that enhances pathogenicity [
31]. The polymicrobial BV-biofilm can be seen with the Gram-stain method in the form of clue cells, which are vaginal epithelial cells covered by layers of adherent Gram-negative and/or -variable cells. Using fluorescence in situ hybridisation (FISH) method, the structure and composition of the biofilm can be studied in more detail [
32], especially considering the combined presence of
G. vaginalis and
A. vaginae. Hardy et al. [
33], similar to that previously described by Swidsinski et al. [
31], have demonstrated that adherent
A. vaginae and
G. vaginalis were visualised in, respectively, 54% and 82% of samples with bacterial biofilm in BV. It was detected that
G. vaginalis accounted for 60% or more and
A. vaginae accounted for 40% or less of the film bacterial composition. It is assumed that
G. vaginalis acts as an initial coloniser to establish early biofilm structures to which secondary colonisers, such as
A. vaginae can attach [
33,
34]. The fact that
G. vaginalis is capable of displacing protective lactobacilli on pre-coated vaginal epithelial cells, is probably related to its ability to promote biofilm formation. In contrast, the other anaerobes, including
A. vaginae, are easily outcompeted by
L. crispatus [
35]. Hardy et al. [
33] also demonstrated the important role of
A. vaginae together with
G. vaginalis in BV-associated biofilm. Interactions among these species within a biofilm are synergistic: these include co-aggregation, metabolic cooperation and increased resistance to antibiotics or host immune responses and have important clinical implications [
36]. The presence of a biofilm—due to increased resistance to treatment—is thought to be one of the possible reasons for the BV recurrence [
37,
38].