Apoptosis in Candida biofilms exposed to amphotericin B

Abstract

Candidabiofilms are resistant to a range of antifungal agents in current clinical use. The basis of this drug resistance is not clear, but in some cases it could be due to the presence of a small number of drug-tolerant or persister cells. In this study, specific staining methods were used to investigate the existence of persisters and apoptosis inCandidabiofilms subjected to different concentrations of amphotericin B. Fluorescein diacetate staining revealed the presence of persisters in biofilms of one of two strains ofCandida albicanstested, and in biofilms ofCandida kruseiandCandida parapsilosis.Caspase activity, indicative of apoptosis, was detected with SR-FLICA and (aspartyl)2-rhodamine 110 fluorochrome-based staining reagents in all of these biofilms. The general inhibitor of mammalian caspases, Z-VAD-FMK, when used at a low concentration (2.5 μM), increased the viability of drug-treated biofilms up to 11.5-fold (P<0.001 %). Seven specific caspase inhibitors had different effects onC. albicansbiofilm viability, but inhibitors of caspases-1, −9, −5, −3 and −2 all significantly increased cell survival (40-fold, 8-fold, 3.5-fold, 1.9-fold and 1.7-fold, respectively). However, histone deacetylase (HDA) inhibitors enhanced the activity of amphotericin B for biofilms of all threeCandidaspecies. Sodium butyrate and sodium valproate, for example, when added concurrently with amphotericin B, completely eliminated biofilm populations ofC. albicans. Overall, our results demonstrate an apoptotic process in amphotericin-treated biofilms of threeCandidaspecies. They also indicate that HDA inhibitors can enhance the action of the drug and in some cases even eradicate persister subpopulations, suggesting that histone acetylation might activate apoptosis in these cells.