Host soluble plasma factors increase dual-species Staphylococcus epidermidis and Candida albicans biofilm biomass without enhancing stress tolerance

Staphylococcus epidermidis (S. epidermidis) and Candida albicans (C. albicans) are common members of the human microbiome and opportunistic pathogens, forming mixed biofilms, leading to catheter-associated bloodstream infections or wound infections. They are increasingly difficult to treat, highlighting the urgent need for new antibiofilm strategies. Understanding how host environmental factors affect microbial communities is crucial for their development. Based on our previous findings, we investigated whether higher levels of host soluble factors in human plasma (HP) and freeze-thaw lysed sheep red blood cells (FT-RBC) support the formation of S. epidermidis-C. albicans dual-species biofilm and increase resilience. Tryptic soy broth, RPMI 1640, and Lubbock media with HP or FT-RBC supplementation were used for in vitro biofilm formation. Total biomass, individual microorganisms, key matrix components (carbohydrates, proteins, eDNA), and antimicrobial tolerance were evaluated. Our results showed that although higher concentrations of HP in Lubbock medium support the formation of complex dual-species biofilm biomass, this does not correlate with enhanced antimicrobial tolerance. In contrast, higher adaptive resistance was detected in less heterogeneous biofilms formed under nitrogen-limited conditions in RPMI 1640-supplemented medium. These findings indicate that biofilm resilience is not solely dependent on biomass amount and complexity, underscoring the need to better understand host-biofilm interactions.