Coassociation between Group B Streptococcus and Candida albicans Promotes Interactions with Vaginal Epithelium

Group B Streptococcus (GBS) is a leading cause of neonatal sepsis, pneumonia and meningitis worldwide. In the majority of cases, GBS is transmitted vertically from mother to neonate, making maternal vaginal colonisation a key risk factor for neonatal disease. The fungus Candida albicans is an opportunistic pathogen of the female genitourinary tract, and the causative agent of vaginal thrush. Carriage of C. albicans has been shown to be an independent risk factor for vaginal colonisation by GBS. However, the nature of interactions between these two microbes is poorly understood. This study provides evidence of a reciprocal, synergistic interplay between GBS and C. albicans that may serve to promote their co-colonisation of the vaginal mucosa. GBS strains NEM316 (serotype III) and 515 (Ia) are shown to physically interact with C. albicans, with bacteria exhibiting tropism for candidal hyphal filaments. This interaction enhances association levels of both microbes with vaginal epithelial cell line VK2/E6E7. The ability of GBS to co-associate with C. albicans is dependent upon expression of hyphal-specific adhesin Als3. In turn, expression of GBS antigen I/II family adhesins (Bsp polypeptides) facilitates this co-association and confers upon surrogate Lactococcus lactis the capacity to exhibit enhanced interactions with C. albicans on vaginal epithelium. As genitourinary tract colonisation is an essential first step in the pathogenesis of GBS and C. albicans, the co-association mechanism reported here may have important implications for risk of disease involving both of these pathogens.