entration of Lactobacillus species in the vaginal environment and an increase in the concentration of G. vaginalis and many other bacterial genera . This shift in microbiota is reflected in quantifiable changes in vaginal smear Gram stains (GS) as measured by the Nugent Score (NS) . Women with Lactobacillus dominated microbiota have NS of 0–3 while women with BV have NS of 7–10. It is important to keep in mind that many women with BV as defined by NS are totally asymptomatic and for this reason some investigators in the field believe this represents a normal variant of the vaginal microbiota . Nevertheless, the microbiota associated with BV as defined by GS pattern is associated with a number of serious medical sequelae including preterm delivery , . A reduction in the concentration of Lactobacillus species leads to an increase in vaginal pH and a deterioration in immune response to sexually transmitted viral infections including HIV . Although the natural history of the microbial communities associated with BV is not yet fully understood , several studies suggest that the condition can be sexually transmitted ,  and that Gardnerella vaginalis may be the etiologic agent . In contrast to the latter assertion, G. vaginalis is also commonly detected in vaginal samples of women with GS-defined normal vaginal microbiota, albeit, at significantly lower concentrations than in GS-defined BV , , .
Phenotypic and genomic analyses of G. vaginalis isolates suggest that, in addition to low concentration, the conflicting observation of the presence of this species in both normal (or asymptomatic) and BV (or symptomatic) women may be rationalized by the existence of different strains of G. vaginalis, i.e. avirulent commensal strains colonize normal women while more-virulent strains may be infecting BV patients. This idea is supported by phenotypic analyses that show biofilm formation is a virulence trait of G. vaginalis isolates and the ability to form biofilms is associated with BV . In addition, a recent genomic study showed that a G. vaginalis isolate from a GS-defined BV patient, differed from an isolate from a GS-defined normal patient by having the capacity to form tightly adherent biofilms on vaginal epithelial cells . Genomic analysis of three G. vaginalis strains, two isolated from GS-defined BV patients and one from a GS-defined normal patient, showed that the GS-defined BV-associated strains produce proteins that are not found in the strain isolated from the GS-defined normal patient . Moreover, another study of three G. vaginalis isolates revealed that two of the three isolates were able to produce sialidase, an enzyme associated with adverse pregnancy outcome in GS-defined BV patients , .
Piot et al. introduced a way to define G. vaginalis biotypes using enzymatic assays for lipase, hippurate hydrolysis and β-galactosidase activities , and defined eight biotypes. However, since eight (23) is the maximum number of different types that can be defined using such an approach, the results may have reached that number not because the biotyping scheme is able to distinguish among all potential strains, but because the approach reached it's limit by finding all eight possible patterns of expression among the isolates. Hence, one cannot tell from these results whether in fact there may be more biotypes. Regardless, given the great diversity in human-host microbial communities, a new approach that has the potential to distinguish more biotypes may indeed reveal more types of G. vaginalis.
We explored the diversity and sexual transmissibility of G. vaginalis by examining the sequence variation and distribution of 65,710 G. vaginalis 16S rRNA pyrosequencing reads that were PCR-amplified from vaginal samples of 35 GS-defined BV, 5 GS-defined intermediate and 8 GS-defined normal women and from penile skin and urethral samples obtained from their male sexual partners. To identify high quality G. vaginalis sequences in our pyrosequencing libraries, and to minimize variation due to pyrosequencing errors, we performed a stringent search aga