CO2 enhances the formation, nutrient scavenging and drug resistance properties of C. albicans biofilms

Abstract

C. albicans is the predominant human fungal pathogen and frequently colonises medical devices, such as voice prostheses, as a biofilm. It is a dimorphic yeast that can switch between yeast and hyphal forms in response to environmental cues, a property that is essential during biofilm establishment and maturation. One such cue is the elevation of CO2 levels, as observed in exhaled breath for example. However, despite the clear medical relevance, the effect of CO2 on C. albicans biofilm growth has not been investigated to date. Here we show that physiologically relevant CO2 elevation enhances each stage of the C. albicans biofilm-forming process: from attachment through maturation to dispersion. The effects of CO2 are mediated via the Ras/cAMP/PKA signalling pathway and the central biofilm regulators Efg1, Brg1, Bcr1 and Ndt80. Biofilms grown under elevated CO2 conditions also exhibit increased azole resistance, increased Sef1-dependent iron scavenging and enhanced glucose uptake to support their rapid growth. These findings suggest that C. albicans has evolved to utilise the CO2 signal to promote biofilm formation within the host. We investigate the possibility of targeting CO2-activated processes and propose 2-deoxyglucose as a drug that may be repurposed to prevent C. albicans biofilm formation on medical airway management implants. We thus characterise the mechanisms by which CO2 promotes C. albicans biofilm formation and suggest new approaches for future preventative strategies.

Impaired amino acid uptake leads to global metabolic imbalance of Candida albicans biofilms

“…At later stages, we observed a dramatic decrease in gene expression of several metabolic pathways, but with a particular increase in amino acid permease genes that are likely required to maintain intracellular amino acid homeostasis. The up-regulation of amino acid and peptide transporters seems to be a conserved feature of microbial biofilms reported for gram-negative and gram-positive bacteria, yeasts, and filamentous fungi 5 , 10 , 16 , 34 – 37 . Interestingly, previous work by Traven et al showed that genes for amino acid and fermentative metabolism were highly expressed in the upper cell layers of S. cerevisiae colonies compared to the lower structures.…”

“…Interestingly, the deletion of STP2 altered transcription of non-amino acid transporters, including nucleotide and glucose importers with carbohydrate transport being a significantly up-regulated GO term in stp2 Δ cells compared to the wild type. Recently, biofilm growth under elevated CO 2 conditions was connected to enhanced glucose uptake via the same set of glucose importers 16 .…”

“…Furthermore, robust biofilm growth is associated with an increased ability to metabolize certain amino acids instead of chained carbohydrates 13 . Indeed, several amino acid permease genes like CAN2 are among the most induced transcripts during biofilm growth 5 , 14 – 16 . In a recent study, we described the important role of the transcriptional regulator of amino acid permeases, Stp2, in adherence and biofilm maturation 17 .…”

The Pga59 cell wall protein is an amyloid forming protein involved in adhesion and biofilm establishment in the pathogenic yeast Candida albicans

“…Biofilm formation is a process tightly regulated in C. albicans that occurs in response to several environmental clues such as surface contact, CO 2 levels or nutrient starvation [18][19][20] . This biological process is orchestrated by a « core » of nine transcription factors that finely regulate lipid metabolism, filamentation and above all adhesion 21,22 .…”

Supporting the strategic pillars of translational research in biofilms

“…This modified polymer can drastically reduce the ability of bacteria to colonise these surfaces providing a scalable new alternative for the functionalisation of catheters, dressings and in-dwelling medical devices to existing strategies, such as the in situ generation of silver nanoparticles 3 . In another study, Pentland et al 4 show that C. albicans uses CO 2 found in the host environment to promote its ability to colonise and compete for nutrients. These findings reveal that the design of therapeutic interventions targeting CO 2 utilisation could be used as a new approach to prevent C. albicans biofilm formation in high CO 2 environments paving the way for new therapeutic approaches against infections caused by this organism.…”