Secreted factors of Staphylococcus aureus promote co-invasion with Candida albicans by inducing hypha formation and invasion

Interkingdom interactions between Candida albicans and Staphylococcus aureus promote lethal dissemination of the bacterium. During this process, C. albicans hypha invasion aids S. aureus dissemination through Als1p/Als3p-facilitated co-invasion. The effects of S. aureus on C. albicans hypha formation and invasion are, however, unknown. In this study, we used both liquid mDMEM-DMP as well as a previously constructed semi-solid adaptation of the medium (mDMEM-DMPA) to study the effects of C. albicans/S. aureus co-culturing on hypha formation and invasion. Semi-solid-based co-culturing significantly increased colony size and generally increased hypha invasion. Liquid growth-based time-lapse microscopy showed that S. aureus significantly promoted both C. albicans hypha length and elongation rate. Further semi-solid-based growth results revealed that >3 kDa-secreted S. aureus factors were accountable for the increase in C. albicans hypha growth. A newly constructed in vitro assay confirmed the co-invasion of S. aureus during co-culturing and showed that deletion of C. albicans Als1p/Als3p abolished the co-invasion of S. aureus during co-culturing. In conclusion, our study shows that S. aureus affects C. albicans virulence by actively stimulating C. albicans hypha extension through the production of, presently unknown, secreted factors and sequentially using hypha proteins Als1p and Als3p to co-invade. Therefore, S. aureus can stimulate C. albicans epithelial invasion even prior to attaching to its hyphae, providing the foundation for subsequent co-invasion.IMPORTANCEEpithelial barriers normally protect against invasion and systemic infection by S. aureus, but frequently, such infections occur without a port of entry. One route of S. aureus epithelial traversal is through co-invasion with the highly invasive Candida albicans. Understanding this interaction in detail is of high importance in view of the prevention of these infections. Our study shows how the S. aureus and C. albicans interaction results in mutual benefit. S. aureus appeared to affect C. albicans virulence by actively stimulating C. albicans hypha extension through the production of, presently unknown, secreted factors and sequentially using hypha proteins Als1p and Als3p to bind to the hyphae and co-invade. These insights are important from a microbial ecological perspective and offer important potential targets for interfering with the interaction and reducing the virulence of these opportunistic pathogens.