Highlights

• Vesicles from Biofilm and non-biofilm Candida populations contain significantly different components
• Mutants unable to produce vesicles become sensitive to treatment with fluconazole, a phenotype that can be complemented
• resistance to fluconazole seems to be associated with functional protein delivery by the vesicles (putative glycanosyltransferase (Phr1) and putative endo-beta-D-glucosidase (Sun41))

Summary

Treating biofilms with drugs has always been difficult and as research into the field gets more mature, the causes of antimicrobial resistance in biofilms become more numerous. This paper takes an interesting look at Candida albicans biofilms and the role that extracellular vesicles play in promoting resistance to fluconazole, an important antifungal. Through careful use of mutants and complementation protocols (including adding purified vesicles to biofilm cultures), the research teams was able to find an interesting mechanism by which vesicles contribute to fluconazole resistance. In essence, the vesicles were used to share specific functional proteins throughout the biofilm. No vesicles meant no resistance and vesicles lacking functional proteins also did not confer resistance. The functional proteins in question were a putative glycanosyltransferase (Phr1) and a putative endo-beta-D-glucosidase (Sun41), which act in the glucan modification pathway. The authors suggest that understanding this and using extracellular vesicle based therapies could be used to circumvent resistance in the future. Of course, for this to work, vesicles would have to be the source of fluconazole resistance in the clinic which has yet to be shown but could be an exciting follow up to this work.

Abstract

Cells from all kingdoms of life produce extracellular vesicles (EVs). Their cargo is protected from the environment by the surrounding lipid bilayer. EVs from many organisms have been shown to function in cell-cell communication, relaying signals that impact metazoan development, microbial quorum sensing, and pathogenic host-microbe interactions. Here, we have investigated the production and functional activities of EVs in a surface-associated microbial community or biofilm of the fungal pathogen Candida albicans. Crowded communities like biofilms are a context in which EVs are likely to function. Biofilms are noteworthy because they are encased in an extracellular polymeric matrix and because biofilm cells exhibit extreme tolerance to antimicrobial compounds. We found that biofilm EVs are distinct from those produced by free-living planktonic cells and display strong parallels in composition to biofilm matrix material. The functions of biofilm EVs were delineated with a panel of mutants defective in orthologs of endosomal sorting complexes required for transport (ESCRT) subunits, which are required for normal EV production in diverse eukaryotes. Most ESCRT-defective mutations caused reduced biofilm EV production, reduced matrix polysaccharide levels, and greatly increased sensitivity to the antifungal drug fluconazole. Matrix accumulation and drug hypersensitivity of ESCRT mutants were reversed by addition of wild-type (WT) biofilm EVs. Vesicle complementation showed that biofilm EV function derives from specific cargo proteins. Our studies indicate that C. albicans biofilm EVs have a pivotal role in matrix production and biofilm drug resistance. Biofilm matrix synthesis is a community enterprise; prior studies of mixed cell biofilms have demonstrated extracellular complementation. Therefore, EVs function not only in cell-cell communication but also in the sharing of microbial community resources.

Reference

Zarnowski R, Sanchez H, Covelli AS, Dominguez E, Jaromin A, Berhardt J, et al. (2018) Candida albicans biofilm–induced vesicles confer drug resistance through matrix biogenesis. PLoS Biol 16(10): e2006872. https://doi.org/10.1371/journal.pbio.2006872