Histatin-induced alterations in Candida albicans: a microscopic and submicroscopic comparison

Inhibitory Effects and Mechanism of Action of Elsinochrome A on Candida albicans and Its Biofilm

Biofilm-associated Candida albicans infections, the leading cause of invasive candidiasis, can cause high mortality rates in immunocompromised patients. Photodynamic antimicrobial chemotherapy (PACT) is a promising approach for controlling infections caused by biofilm-associated C. albicans. This study shows the effect of Elsinochrome A (EA) against different stages of C. albicans biofilms in vitro by XTT reduction assay and crystal violet staining. The mechanism of action of EA on C. albicans biofilm was analyzed with flow cytometry, confocal laser microscopy, and the Real-Time Quantitative Reverse Transcription PCR (qRT-PCR). EA-mediated PACT significantly reduced the viability of C. albicans, with an inhibition rate on biofilm of 89.38% under a concentration of 32 μg/mL EA. We found that EA could not only inhibit the adhesion of C. albicans in the early stage of biofilm formation, but that it also had good effects on pre-formed mature biofilms with a clearance rate of 35.16%. It was observed that EA-mediated PACT promotes the production of a large amount of reactive oxygen species (ROS) in C. albicans and down-regulates the intracellular expression of oxidative-stress-related genes, which further disrupted the permeability of cell membranes, leading to mitochondrial and nuclear damage. These results indicate that EA has good photodynamic antagonizing activity against the C. albicans biofilm, and potential clinical value.

In vitro antifungal activity of baicalin against Candida albicans biofilms via apoptotic induction

The aim of this study was to investigate the antifungal activity of baicalin and its potential mechanism of action against Candida albicans biofilms. The standard techniques including microdilution method and checkerboard assay were employed to evaluate the susceptibilities of baicalin alone and in combination with fluconazole against planktonic and biofilm cells of C. albicans. Transmission electron microscope (TEM), scanning electron microscope (SEM), fluorescent microscope and flow cytometry were used to assess the apoptotic incidences induced by baicalin in biofilm cells. The expressions of four genes (RAS1, CAP1, PDE2 and TPK1) related to Ras-cAMP-PKA pathway were also analyzed by quantitative reverse transcription polymerase chain reaction (qRT-PCR). The results showed that minimum inhibitory concentration (MIC) and sessile minimum inhibitory concentration (SMIC50) of baicalin were 500 and 2000 μg/mL with fractional inhibitory concentration indexs (FICIs) ranging from 0.28 to 0.75. A series of events related to apoptosis were observed in baicalin-treated C. albicans biofilms, including extensive chromatin condensation along the nuclear envelope, ROS accumulation, MMP reduction, PS externalization, nuclear fragmentation, chromatin condensation, metacaspase activation and Cyt C release. Additionally, the expressions of RAS1 and TPK1 were up-regulated by 3.2 and 2.9 folds respectively, while those of CAP1 and PDE2 were down-regulated by 3.3 and 6.6 folds respectively after exposure to baicalin in biofilm cells. In conclusion, baicalin can suppress the development of C. albicans biofilms most likely due to inducing cell death via apoptosis.

Honey flavonoids inhibit Candida albicans morphogenesis by affecting DNA behavior and mitochondrial function

AIM

Candida albicans is a pathogenic yeast, which forms a range of polarized and expanded cell shapes. We aimed to determine the correlation between honey extract (HFE) activity and changes in C. albicans cell cycle, morphology and subcellular organelles.

MATERIALS & METHODS

HFE anticandidal properties were investigated using flow cytometry and scanning electron microscopy.

RESULTS

Flow cytometry and scanning electron microscopy analyses indicated that HFE may inhibit the growth of the three phenotypes displayed by C. albicans and reduce infection by affecting membrane integrity. HFE affects hyphal transition by reducing the G0/G1 phase and increasing the G2/M phase. Conversely, yeast and pseudohyphae do not appear to be affected. Modifications of vacuolization and mitochondrial activity, during yeast-hypha transition establish the involvement of vacuole and mitochondria.

CONCLUSION

HFE improved mitochondrial functionality and reduced the vacuolization, modifying the branching process associated with virulence. It is hypothesized that HFE induces changes in cell cycle progress, membrane integrity, mitochondrial function and biogenesis.

The antimicrobial peptide, psacotheasin induces reactive oxygen species and triggers apoptosis in Candida albicans

Previously, the antimicrobial effects and membrane-active action of psacotheasin in Candida albicans were investigated. In this study, we have further found that a series of characteristic cellular changes of apoptosis in C. albicans can be induced by the accumulation of intracellular reactive oxygen species, specifically hydroxyl radicals, the well-known important regulators of apoptosis. Cells treated with psacotheasin showed diagnostic markers in yeast apoptosis at early stages: phosphatidylserine externalization from the inner to the outer membrane surface, visualized by Annexin V-staining; mitochondrial membrane depolarization, observed by DiOC6(3) staining; and increase of metacaspase activity, measured using the CaspACE FITC-VAD-FMK. Moreover, DNA fragmentation and condensation also revealed apoptotic phenomena at late stages through the TUNEL assay staining and DAPI staining, respectively. Taken together, our findings suggest that psacotheasin possess an antifungal property in C. albicans via apoptosis as another mode of action.

Soft X-ray tomography of phenotypic switching and the cellular response to antifungal peptoids in Candida albicans

The opportunistic pathogen Candida albicans can undergo phenotypic switching between a benign, unicellular phenotype and an invasive, multicellular form that causes candidiasis. Increasingly, strains of Candida are becoming resistant to antifungal drugs, making the treatment of candidiasis difficult, especially in immunocompromised or critically ill patients. Consequently, there is a pressing need to develop new drugs that circumvent fungal drug-resistance mechanisms. In this work we used soft X-ray tomography to image the subcellular changes that occur as a consequence of both phenotypic switching and of treating C. albicans with antifungal peptoids, a class of candidate therapeutics unaffected by drug resistance mechanisms. Peptoid treatment suppressed formation of the pathogenic hyphal phenotype and resulted in striking changes in cell and organelle morphology, most dramatically in the nucleus and nucleolus, and in the number, size, and location of lipidic bodies. In particular, peptoid treatment was seen to cause the inclusion of lipidic bodies into the nucleus.

The antimicrobial peptide histatin-5 causes a spatially restricted disruption on the Candida albicans surface, allowing rapid entry of the peptide into the cytoplasm

Antimicrobial peptides play an important role in host defense against microbial pathogens. Their high cationic charge and strong amphipathic structure allow them to bind to the anionic microbial cell membrane and disrupt the membrane bilayer by forming pores or channels. In contrast to the classical pore-forming peptides, studies on histatin-5 (Hst-5) have suggested that the peptide is transported into the cytoplasm of Candida albicans in a non-lytic manner, and cytoplasmic Hst-5 exerts its candicidal activities on various intracellular targets, consistent with its weak amphipathic structure. To understand how Hst-5 is internalized, we investigated the localization of FITC-conjugated Hst-5. We find that Hst-5 is internalized into the vacuole through receptor-mediated endocytosis at low extracellular Hst-5 concentrations, whereas under higher physiological concentrations, Hst-5 is translocated into the cytoplasm through a mechanism that requires a high cationic charge on Hst-5. At intermediate concentrations, two cell populations with distinct Hst-5 localizations were observed. By cell sorting, we show that cells with vacuolar localization of Hst-5 survived, while none of the cells with cytoplasmic Hst-5 formed colonies. Surprisingly, extracellular Hst-5, upon cell surface binding, induces a perturbation on the cell surface, as visualized by an immediate and rapid internalization of Hst-5 and propidium iodide or rhodamine B into the cytoplasm from the site using time-lapse microscopy, and a concurrent rapid expansion of the vacuole. Thus, the formation of a spatially restricted site in the plasma membrane causes the initial injury to C. albicans and offers a mechanism for its internalization into the cytoplasm. Our study suggests that, unlike classical channel-forming antimicrobial peptides, action of Hst-5 requires an energized membrane and causes localized disruptions on the plasma membrane of the yeast. This mechanism of cell membrane disruption may provide species-specific killing with minimal damage to microflora and the host and may be used by many other antimicrobial peptides.

Drug-induced apoptosis in yeast

In order to alter the impact of diseases on human society, drug development has been one of the most invested research fields. Nowadays, cancer and infectious diseases are leading targets for the design of effective drugs, in which the primary mechanism of action relies on the modulation of programmed cell death (PCD). Due to the high degree of conservation of basic cellular processes between yeast and higher eukaryotes, and to the existence of an ancestral PCD machinery in yeast, yeasts are an attractive tool for the study of affected pathways that give insights into the mode of action of both antitumour and antifungal drugs. Therefore, we covered some of the leading reports on drug-induced apoptosis in yeast, revealing that in common with mammalian cells, antitumour drugs induce apoptosis through reactive oxygen species (ROS) generation and altered mitochondrial functions. The evidence presented suggests that yeasts may be a powerful model for the screening/development of PCD-directed drugs, overcoming the problem of cellular specificity in the design of antitumour drugs, but also enabling the design of efficient antifungal drugs, targeted to fungal-specific apoptotic regulators that do not have major consequences for human cells.