Sialidase variability in Gardnerella: genetics, taxonomy, function, and clinical presentation

Bacterial vaginosis (BV) is the most common vaginal condition among women of reproductive age. Despite over six decades of research, the pathogenesis and etiology of BV remain largely unknown. Members of the Gardnerella genus are common components of the healthy vaginal microbiota, although their uncontrolled proliferation plays a key role in BV pathogenesis. Recent taxonomic revisions have highlighted extensive genetic and functional diversity among Gardnerella spp., but the relationship between genetics, taxonomy, and virulence remains elusive. Sialidases are key virulence factors in BV, degrading the protective epithelial mucus layer, facilitating colonization and releasing carbon and nitrogen sources. However, robust clinical evidence linking sialidases to increased BV risk remains limited. Here, we analyzed clinical Gardnerella isolates from BV-positive (75%) and BV-negative (25%) samples (n = 66) collected at the University Hospital of Montpellier, France. We inferred phylogenetic relationships, characterised the nanH sialidase gene repertoire, quantified in vitro sialidase activity, and evaluated antibiotic resistance profiles. Most isolates belonged to Gardnerella clades 1, 2, and 4 (63.6%, 19.7%, and 16.7%, respectively). The three nanH genes were unevenly distributed across clades: nanH1 was most common ([~]80%), highly prevalent in clades 1-3; nanH2 was rare (6%); and nanH3 ([~]38%) was predominantly found in clade 2 - typically co-occurring with nanH1. Sialidase activity varied both between and within clades, and was only weakly associated with the nanH gene repertoire. Neither taxonomy nor sialidase-associated traits were reliable predictors of BV status, and we found no clear link between taxonomy and antibiotic resistance. Together, these results reveal substantial discordance in the genotype-phenotype relationship in Gardnerella, suggesting epigenetic, physiological and/or ecological modulation. Our findings highlight the need for a better understanding of bacterial physiology, plasticity, and ecology to explain microbial traits of clinical importance.