Pseudomonas aeruginosa increases the susceptibility of Candida albicans to amphotericin B in dual species biofilms

Biofilms are the leading cause of nosocomial infections, and are hard to eradicate due to their inherent antimicrobial resistance. Candida albicans is the leading cause of nosocomial fungal infections, and is frequently co-isolated with the bacterium Pseudomonas aeruginosa from biofilms in the Cystic Fibrosis lung and severe burn wounds. The presence of C. albicans in multi-species biofilms is associated with enhanced antibacterial resistance, which is largely mediated through fungal extracellular carbohydrates sequestering the antibiotics. However, significantly less is known regarding the impact of polymicrobial biofilms on antifungal resistance. Here we show that, in dual species biofilms, P. aeruginosa enhances the sensitivity of C. albicans to amphotericin B, an effect that was biofilm specific. Transcriptional analysis combined with gene ontology enrichment analysis identified several C. albicans processes associated with oxidative stress to be differentially regulated in dual species biofilms, suggesting that P. aeruginosa exerts oxidative stress on C. albicans, likely through the actions of secreted phenazines. However, the mitochondrial superoxide dismutase SOD2 was significantly downregulated in the presence of P. aeruginosa. Monospecies biofilms of the sod2{Delta} mutant were more susceptible to amphotericin B, and the susceptibility of these biofilms was further enhanced by the addition of exogenous phenazines. Therefore, we propose that in dual species biofilms, P. aeruginosa simultaneously induces mitochondrial oxidative stress, whilst downregulating key detoxification enzymes, which prevent C. albicans mounting an appropriate oxidative stress response to amphotericin B, leading to fungal cell death. This work highlights the importance of understanding the impact of polymicrobial interactions on antimicrobial susceptibility. ImportanceBiofilms are aggregates of cells encased in an extracellular matrix, and are responsible for the majority of infections in hospitals. The Gram-negative bacterium Pseudomonas aeruginosa, and the fungal pathogen Candida albicans are frequently co-isolated from biofilms in the Cystic Fibrosis lung, and in burn wounds. Here we show that in these biofilms, P. aeruginosa secreted phenazines induce mitochondrial reactive oxygen species stress, which enhances the sensitivity of C. albicans to the antifungal amphotericin B. This work highlights the importance of understanding the impact of polymicrobial interactions on antimicrobial susceptibility.