Histatin 5-spermidine conjugates have enhanced fungicidal activity and efficacy as a topical therapeutic for oral candidiasis

Pathophysiological microenvironments in oral candidiasis

Oral candidiasis (OC), a prevalent opportunistic infection of the oral mucosa, presents a considerable health challenge, particularly in individuals with compromised immune responses, advanced age, and local predisposing conditions. A considerable part of the population carries Candida in the oral cavity, but only few develop OC. Therefore, the pathogenesis of OC may depend on factors other than the attributes of the fungus, such as host factors and other predisposing factors. Mucosal trauma and inflammation compromise epithelial integrity, fostering a conducive environment for fungal invasion. Molecular insights into the immunocompromised state reveal dysregulation in innate and adaptive immunity, creating a permissive environment for Candida proliferation. Detailed examination of Candida species (spp.) and their virulence factors uncovers a nuanced understanding beyond traditional C. albicans focus, which embrace diverse Candida spp. and their strategies, influencing adhesion, invasion, immune evasion, and biofilm formation. Understanding the pathophysiological microenvironments in OC is crucial for the development of targeted therapeutic interventions. This review aims to unravel the diverse pathophysiological microenvironments influencing OC development focusing on microbial, host, and predisposing factors, and considers Candida resistance to antifungal therapy. The comprehensive approach offers a refined perspective on OC, seeking briefly to identify potential therapeutic targets for future effective management.

High iron-mediated increased oral fungal burden, oral-to-gut transmission, and changes to pathogenicity of Candida albicans in oropharyngeal candidiasis

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Histatin 5 Metallopeptides and Their Potential against Candida albicans Pathogenicity and Drug Resistance

Usually caused by Candida albicans, buccal candidiasis begins with the morphological transition between yeast and hyphal cells. Over time and without the correct treatment, it can be disseminated through the bloodstream becoming a systemic infection with high mortality rates. C. albicans already shows resistance against antifungals commonly used in treatments. Therefore, the search for new drugs capable of overcoming antifungal resistance is essential. Histatin 5 (Hst5) is an antimicrobial peptide of the Histatin family, that can be found naturally in human saliva. This peptide presents high antifungal activity against C. albicans. However, Hst5 action can be decreased for interaction with enzymes and metal ions present in the oral cavity. The current work aims to bring a brief review of relevant aspects of the pathogenesis and resistance mechanisms already reported for C. albicans. In addition, are also reported here the main immune responses of the human body and the most common antifungal drugs. Finally, the most important aspects regarding Histatin 5 and the benefits of its interaction with metals are highlighted. The intention of this review is to show the promising use of Hst5 metallopeptides in the development of effective drugs.

Zinc Binding by Histatin 5 Promotes Fungicidal Membrane Disruption in C. albicans and C. glabrata

Histatin 5 (Hst 5) is an antimicrobial peptide produced in human saliva with antifungal activity for opportunistic pathogen Candida albicans. Hst 5 binds to multiple cations including dimerization-inducing zinc (Zn2+), although the function of this capability is incompletely understood. Hst 5 is taken up by C. albicans and acts on intracellular targets under metal-free conditions; however, Zn2+ is abundant in saliva and may functionally affect Hst 5. We hypothesized that Zn2+ binding would induce membrane-disrupting pores through dimerization. Through the use of Hst 5 and two derivatives, P113 (AA 4-15 of Hst 5) and Hst 5ΔMB (AA 1-3 and 15-19 mutated to Glu), we determined that Zn2+ significantly increases killing activity of Hst 5 and P113 for both C. albicans and Candida glabrata. Cell association assays determined that Zn2+ did not impact initial surface binding by the peptides, but Zn2+ did decrease cell association due to active peptide uptake. ATP efflux assays with Zn2+ suggested rapid membrane permeabilization by Hst 5 and P113 and that Zn2+ affinity correlates to higher membrane disruption ability. High-performance liquid chromatography (HPLC) showed that the higher relative Zn2+ affinity of Hst 5 likely promotes dimerization. Together, these results suggest peptide assembly into fungicidal pore structures in the presence of Zn2+, representing a novel mechanism of action that has exciting potential to expand the list of Hst 5-susceptible pathogens.

Chemokine CCL28 Is a Potent Therapeutic Agent for Oropharyngeal Candidiasis

Candida albicans is a commensal organism that causes life-threatening or life-altering opportunistic infections. Treatment of Candida infections is limited by the paucity of antifungal drug classes. Naturally occurring antimicrobial peptides are promising agents for drug development. CCL28 is a CC chemokine that is abundant in saliva and has in vitro antimicrobial activity. In this study, we examine the in vivo Candida killing capacity of CCL28 in oropharyngeal candidiasis as well as the spectrum and mechanism of anti-Candida activity. In the mouse model of oropharyngeal candidiasis, application of wild-type CCL28 reduces oral fungal burden in severely immunodeficient mice without causing excessive inflammation or altering tissue neutrophil recruitment. CCL28 is effective against multiple clinical strains of C. albicans Polyamine protein transporters are not required for CCL28 anti-Candida activity. Both structured and unstructured CCL28 proteins show rapid and sustained fungicidal activity that is superior to that of clinical antifungal agents. Application of wild-type CCL28 to C. albicans results in membrane disruption as measured by solute movement, enzyme leakage, and induction of negative Gaussian curvature on model membranes. Membrane disruption is reduced in CCL28 lacking the functional C-terminal tail. Our results strongly suggest that CCL28 can exert antifungal activity in part via membrane permeation and has potential for development as an anti-Candida therapeutic agent without inflammatory side effects.

Bio- and Nanotechnology as the Key for Clinical Application of Salivary Peptide Histatin: A Necessary Advance

Candida albicans is a common microorganism of human’s microbiota and can be easily found in both respiratory and gastrointestinal tracts as well as in the genitourinary tract. Approximately 30% of people will be infected by C. albicans during their lifetime. Due to its easy adaptation, this microorganism started to present high resistance to antifungal agents which is associated with their indiscriminate use. There are several reports of adaptive mechanisms that this species can present. Some of them are intrinsic alteration in drug targets, secretion of extracellular enzymes to promote host protein degradation and efflux receptors that lead to a diminished action of common antifungal and host’s innate immune response. The current review aims to bring promising alternatives for the treatment of candidiasis caused mainly by C. albicans. One of these alternatives is the use of antifungal peptides (AFPs) from the Histatin family, like histatin-5. Besides that, our focus is to show how nanotechnology can allow the application of these peptides for treatment of this microorganism. In addition, our intention is to show the importance of nanoparticles (NPs) for this purpose, which may be essential in the near future.

Integrating All-Atom and Coarse-Grained Simulations-Toward Understanding of IDPs at Surfaces

We present a scheme for transferring conformational degrees of freedom from all-atom (AA) simulations of an intrinsically disordered protein (IDP) to coarse-grained (CG) Monte Carlo (MC) simulations using conformational swap moves. AA simulations of a single histatin 5 peptide in water were used to obtain a structural ensemble, which is reweighted in a CGMC simulation in the presence of a negatively charged surface. For efficient sampling, the AA trajectory was condensed using two approaches: RMSD clustering (based on the root-mean-square difference in atom positions) and a "naı̈ve" truncation, where only every 100th frame of the trajectory was included in the library. The results show that even libraries with few structures well reproduce the radius of gyration and interaction free energy as functions of the distance from the surface. We further observe that the surface slightly promotes the secondary structure of histatin 5 and more so if using explicit surface charges rather than smeared charges.

Bioactive Peptides Against Fungal Biofilms

Infections caused by invasive fungal biofilms have been widely associated with high morbidity and mortality rates, mainly due to the advent of antibiotic resistance. Moreover, fungal biofilms impose an additional challenge, leading to multidrug resistance. This fact, along with the contamination of medical devices and the limited number of effective antifungal agents available on the market, demonstrates the importance of finding novel drug candidates targeting pathogenic fungal cells and biofilms. In this context, an alternative strategy is the use of antifungal peptides (AFPs) against fungal biofilms. AFPs are considered a group of bioactive molecules with broad-spectrum activities and multiple mechanisms of action that have been widely used as template molecules for drug design strategies aiming at greater specificity and biological efficacy. Among the AFP classes most studied in the context of fungal biofilms, defensins, cathelicidins and histatins have been described. AFPs can also act by preventing the formation of fungal biofilms and eradicating preformed biofilms through mechanisms associated with cell wall perturbation, inhibition of planktonic fungal cells’ adhesion onto surfaces, gene regulation and generation of reactive oxygen species (ROS). Thus, considering the critical scenario imposed by fungal biofilms and associated infections and the application of AFPs as a possible treatment, this review will focus on the most effective AFPs described to date, with a core focus on antibiofilm peptides, as well as their efficacy in vivo, application on surfaces and proposed mechanisms of action.

Iron Chelator Deferasirox Reduces Candida albicans Invasion of Oral Epithelial Cells and Infection Levels in Murine Oropharyngeal Candidiasis

Candida albicans, the causative agent of mucosal infections, including oropharyngeal candidiasis (OPC), as well as bloodstream infections, is becoming increasingly resistant to existing treatment options. In the absence of novel drug candidates, drug repurposing aimed at using existing drugs to treat off-label diseases is a promising strategy. C. albicans requires environmental iron for survival and virulence, while host nutritional immunity deploys iron-binding proteins to sequester iron and reduce fungal growth. Here we evaluated the role of iron limitation using deferasirox (an FDA-approved iron chelator for the treatment of patients with iron overload) during murine OPC and assessed deferasirox-treated C. albicans for its interaction with human oral epithelial (OE) cells, neutrophils, and antimicrobial peptides. Therapeutic deferasirox treatment significantly reduced salivary iron levels, while a nonsignificant reduction in the fungal burden was observed. Preventive treatment that allowed for two additional days of drug administration in our murine model resulted in a significant reduction in the number of C. albicans CFU per gram of tongue tissue, a significant reduction in salivary iron levels, and significantly reduced neutrophil-mediated inflammation. C. albicans cells harvested from the tongues of animals undergoing preventive treatment had the differential expression of 106 genes, including those involved in iron metabolism, adhesion, and the response to host innate immunity. Moreover, deferasirox-treated C. albicans cells had a 2-fold reduction in survival in neutrophil phagosomes (with greater susceptibility to oxidative stress) and reduced adhesion to and invasion of OE cells in vitro Thus, deferasirox treatment has the potential to alleviate OPC by affecting C. albicans gene expression and reducing virulence.

Methodologies for in vitro and in vivo evaluation of efficacy of antifungal and antibiofilm agents and surface coatings against fungal biofilms

Unlike superficial fungal infections of the skin and nails, which are the most common fungal diseases in humans, invasive fungal infections carry high morbidity and mortality, particularly those associated with biofilm formation on indwelling medical devices. Therapeutic management of these complex diseases is often complicated by the rise in resistance to the commonly used antifungal agents. Therefore, the availability of accurate susceptibility testing methods for determining antifungal resistance, as well as discovery of novel antifungal and antibiofilm agents, are key priorities in medical mycology research. To direct advancements in this field, here we present an overview of the methods currently available for determining (i) the susceptibility or resistance of fungal isolates or biofilms to antifungal or antibiofilm compounds and compound combinations; (ii) the in vivo efficacy of antifungal and antibiofilm compounds and compound combinations; and (iii) the in vitro and in vivo performance of anti-infective coatings and materials to prevent fungal biofilm-based infections.

Broth Microdilution In Vitro Screening: An Easy and Fast Method to Detect New Antifungal Compounds

Fungal infections have become an important medical condition in the last decades, but the number of available antifungal drugs is limited. In this scenario, the search for new antifungal drugs is necessary. The protocol reported here details a method to screen peptides for their antifungal properties. It is based on the broth microdilution susceptibility test from the Clinical and Laboratory Standards Institute (CLSI) M27-A3 guidelines with modifications to suit the research of antimicrobial peptides as potential new antifungals. This protocol describes a functional assay to evaluate the activity of antifungal compounds and may be easily modified to suit any particular class of molecules under investigation. Since the assays are performed in 96-well plates using small volumes, a large-scale screening can be completed in a short amount of time, especially if carried out in an automation setting. This procedure illustrates how a standardized and adjustable clinical protocol can help the bench-work pursuit of new molecules to improve the therapy of fungal diseases.

Specific Histidine Residues Confer Histatin Peptides with Copper-Dependent Activity against Candida albicans

The histidine-rich salivary peptides of the histatin family are known to bind copper (Cu) and other metal ions in vitro; however, the details of these interactions are poorly understood, and their implications for in vivo antifungal activity have not been established. Here, we show that the availability of Cu during exposure of Candida albicans to histatin-5 (Hist-5) modulates its antifungal activity. Antifungal susceptibility testing revealed that co-treatment of Hist-5 with Cu improved the EC₅₀ from ∼5 to ∼1 μM, whereas co-treatment with a high-affinity Cu-specific chelator abrogated antifungal activity. Spectrophotometric titrations revealed two previously unrecognized Cu(I)-binding sites with apparent K_d values at pH 7.4, ∼20 nM, and confirmed a high-affinity Cu(II)-binding site at the Hist-5 N-terminus with an apparent K_d of ∼8 pM. Evaluation of a series of His-to-Ala full-length and truncated Hist-5 peptides identified adjacent His residues (bis-His) as critical anchors for Cu(I) binding, with the presence of a third ligand revealed by X-ray absorption spectroscopy. On their own, the truncated peptides were ineffective at inhibiting the growth of C. albicans, but treatment with supplemental Cu resulted in EC₅₀ values down to ∼5 μM, approaching that of full-length Hist-5. The efficacy of the peptides depended on an intact bis-His site and correlated with Cu(I) affinity. Together, these results establish new structure-function relationships linking specific histidine residues with Cu binding affinity and antifungal activity and provide further evidence of the involvement of metals in modulating the biological activity of these antifungal peptides.

Structural Characterization of Histatin 5-Spermidine Conjugates: A Combined Experimental and Theoretical Study

Histatin 5 (Hst5) is a naturally occurring antimicrobial peptide that acts as the first line of defense against oral candidiasis. It has been shown that conjugation of the active Hst5 fragment, Hst5_4-15, and the polyamine spermidine (Spd) improves the candidacidal effect. Knowledge about the structure of these conjugates is, however, very limited. Thus, the aim of this study was to characterize the structural properties of the Hst5_4-15-Spd conjugates by performing atomistic molecular dynamics simulations in combination with small-angle X-ray scattering. It was shown that the Hst5_4-15-Spd conjugates adopt extended and slightly rigid random coil conformations without any secondary structure in aqueous solution. It is hypothesized that the increased fungal killing potential of Hst5_4-15-Spd, in comparison with the Spd-Hst5_4-15 conjugate, is due to the more extended conformations of the former, which cause the bonded Spd molecule to be more accessible for recognition by polyamine transporters in the cell.

Human Salivary Protein Histatin 5 Has Potent Bactericidal Activity against ESKAPE Pathogens

ESKAPE (Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumanni, Pseudomonas aeruginosa, and Enterobacter species) pathogens have characteristic multiple-drug resistance and cause an increasing number of nosocomial infections worldwide. Peptide-based therapeutics to treat ESKAPE infections might be an alternative to conventional antibiotics. Histatin 5 (Hst 5) is a salivary cationic histidine-rich peptide produced only in humans and higher primates. It has high antifungal activity against Candida albicans through an energy-dependent, non-lytic process; but its bactericidal effects are less known. We found Hst 5 has bactericidal activity against S. aureus (60–70% killing) and A. baumannii (85–90% killing) in 10 and 100 mM sodium phosphate buffer (NaPB), while killing of >99% of P. aeruginosa, 60–80% E. cloacae and 20–60% of E. faecium was found in 10 mM NaPB. Hst 5 killed 60% of biofilm cells of P. aeruginosa, but had reduced activity against biofilms of S. aureus and A. baumannii. Hst 5 killed 20% of K. pneumonia biofilm cells but not planktonic cells. Binding and uptake studies using FITC-labeled Hst 5 showed E. faecium and E. cloacae killing required Hst 5 internalization and was energy dependent, while bactericidal activity was rapid against P. aeruginosa and A. baumannii suggesting membrane disruption. Hst 5-mediated killing of S. aureus was both non-lytic and energy independent. Additionally, we found that spermidine conjugated Hst 5 (Hst5-Spd) had improved killing activity against E. faecium, E. cloacae, and A. baumannii. Hst 5 or its derivative has antibacterial activity against five out of six ESKAPE pathogens and may be an alternative treatment for these infections.

Endogenous antimicrobial factors in the treatment of infectious diseases

Nowadays, a number of antibiotic-resistant bacteria strains is increasing. It is a serious clinical problem and poses a threat to the effectiveness of conventional antibiotic therapy. Thus, scientists are constantly seeking new alternatives for treatment of infectious diseases. There are some natural endogenous factors, which possess antimicrobial activities against a large number of microorganisms, including both Gram-positive and Gram-negative bacteria, viruses and fungi. These factors are present in all eukaryotic organisms and constitute an essential element of their immune system. A large number of in vitro and in vivo models have been used to show the activity of antimicrobial factors, and only few studies have been conducted on people. Results indicate that administration of these molecules is therapeutically beneficial. This review summarizes knowledge of selected endogenous antimicrobial agents, such as cathelicidins, defensins, histatins, lysozyme and lactoferrin. We also discuss potential uses of these factors in the treatment of infectious diseases.

Inhibition of Candida albicans Biofilm Formation by the Synthetic Lactoferricin Derived Peptide hLF1-11

The aim of this study was to evaluate the in vitro activity of the synthetic peptide hLF1-11 against biofilm produced by clinical isolates of Candida albicans with different fluconazole susceptibility. The antibiofilm activity of the peptide hLF1-11 was assessed in terms of reduction of biofilm cellular density, metabolic activity and sessile cell viability. The extent of morphogenesis in hLF1-11 treated and untreated biofilms was also investigated microscopically. Transcription levels of genes related to cell adhesion, hyphal development and extracellular matrix production were analysed by qRT-PCR in hLF1-11 treated and untreated biofilms. Exogenous dibutyryl-cAMP (db-cAMP) was used to rescue morphogenesis in cells exposed to the peptide. The results revealed that hLF1-11 exhibited an inhibitory effect on biofilm formation by all C. albicans isolates tested in a dose-dependent manner, regardless of their fluconazole susceptibility. Visual inspection of treated or untreated biofilm cells with an inverted microscope revealed a significant reduction in hyphal formation by hLF1-11 treated cells, as early as 3 hours of incubation. Moreover, hLF1-11 showed a reduced activity on preadherent cells. hLF1-11 induced the down-regulation of biofilm and hyphal-associated genes, which were predominantly regulated via the Ras1-cAMP-Efg1 pathway. Indeed, exogenous db-cAMP restored morphogenesis in hLF1-11 treated cells. The hLF1-11 peptide significantly inhibited biofilm formation by C. albicans mainly at early stages, interfering with biofilm cellular density and metabolic activity, and affected morphogenesis through the Ras1-cAMP-Efg1 pathway. Our findings provide the first evidence that hLF1-11 could represent a potential candidate for the prevention of biofilm formation by C. albicans.

pH Dependent Antimicrobial Peptides and Proteins, Their Mechanisms of Action and Potential as Therapeutic Agents

Antimicrobial peptides (AMPs) are potent antibiotics of the innate immune system that have been extensively investigated as a potential solution to the global problem of infectious diseases caused by pathogenic microbes. A group of AMPs that are increasingly being reported are those that utilise pH dependent antimicrobial mechanisms, and here we review research into this area. This review shows that these antimicrobial molecules are produced by a diverse spectrum of creatures, including vertebrates and invertebrates, and are primarily cationic, although a number of anionic examples are known. Some of these molecules exhibit high pH optima for their antimicrobial activity but in most cases, these AMPs show activity against microbes that present low pH optima, which reflects the acidic pH generally found at their sites of action, particularly the skin. The modes of action used by these molecules are based on a number of major structure/function relationships, which include metal ion binding, changes to net charge and conformational plasticity, and primarily involve the protonation of histidine, aspartic acid and glutamic acid residues at low pH. The pH dependent activity of pore forming antimicrobial proteins involves mechanisms that generally differ fundamentally to those used by pH dependent AMPs, which can be described by the carpet, toroidal pore and barrel-stave pore models of membrane interaction. A number of pH dependent AMPs and antimicrobial proteins have been developed for medical purposes and have successfully completed clinical trials, including kappacins, LL-37, histatins and lactoferrin, along with a number of their derivatives. Major examples of the therapeutic application of these antimicrobial molecules include wound healing as well as the treatment of multiple cancers and infections due to viruses, bacteria and fungi. In general, these applications involve topical administration, such as the use of mouth washes, cream formulations and hydrogel delivery systems. Nonetheless, many pH dependent AMPs and antimicrobial proteins have yet to be fully characterized and these molecules, as a whole, represent an untapped source of novel biologically active agents that could aid fulfillment of the urgent need for alternatives to conventional antibiotics, helping to avert a return to the pre-antibiotic era.

Natural and synthetic peptides with antifungal activity

In recent years, the increase of invasive fungal infections and the emergence of antifungal resistance stressed the need for new antifungal drugs. Peptides have shown to be good candidates for the development of alternative antimicrobial agents through high-throughput screening, and subsequent optimization according to a rational approach. This review presents a brief overview on antifungal natural peptides of different sources (animals, plants, micro-organisms), peptide fragments derived by proteolytic cleavage of precursor physiological proteins (cryptides), synthetic unnatural peptides and peptide derivatives. Antifungal peptides are schematically reported based on their structure, antifungal spectrum and reported effects. Natural or synthetic peptides and their modified derivatives may represent the basis for new compounds active against fungal infections.

Design and Application of Antimicrobial Peptide Conjugates

Antimicrobial peptides (AMPs) are an interesting class of antibiotics characterized by their unique antibiotic activity and lower propensity for developing resistance compared to common antibiotics. They belong to the class of membrane-active peptides and usually act selectively against bacteria, fungi and protozoans. AMPs, but also peptide conjugates containing AMPs, have come more and more into the focus of research during the last few years. Within this article, recent work on AMP conjugates is reviewed. Different aspects will be highlighted as a combination of AMPs with antibiotics or organometallic compounds aiming to increase antibacterial activity or target selectivity, conjugation with photosensitizers for improving photodynamic therapy (PDT) or the attachment to particles, to name only a few. Owing to the enormous resonance of antimicrobial conjugates in the literature so far, this research topic seems to be very attractive to different scientific fields, like medicine, biology, biochemistry or chemistry.

Histatin peptides: Pharmacological functions and their applications in dentistry

There are many human oral antimicrobial peptides responsible for playing important roles including maintenance, repairing of oral tissues (hard or soft) and defense against oral microbes. In this review we have highlighted the biochemistry, physiology and proteomics of human oral histatin peptides, secreted from parotid and submandibular salivary glands in human. The significance of these peptides includes capability for ionic binding that can kill fungal Candida albicans. They have histidine rich amino acid sequences (7-12 family members; corresponding to residues 12-24, 13-24, 12-25, 13-25, 5-11, and 5-12, respectively) for Histatin-3. However, Histatin-3 can be synthesized proteolytically from histatin 5 or 6. Due to their fungicidal response and high biocompatibility (little or no toxicity), these peptides can be considered as therapeutic agents with most probable applications for example, artificial saliva for denture wearers and salivary gland dysfunction conditions. The objectives of current article are to explore the human histatin peptides for its types, chemical and biological aspects. In addition, the potential for therapeutic bio-dental applications has been elaborated.

Candida glabrata Binding to Candida albicans Hyphae Enables Its Development in Oropharyngeal Candidiasis

Pathogenic mechanisms of Candida glabrata in oral candidiasis, especially because of its inability to form hyphae, are understudied. Since both Candida albicans and C. glabrata are frequently co-isolated in oropharyngeal candidiasis (OPC), we examined their co-adhesion in vitro and observed adhesion of C. glabrata only to C. albicans hyphae microscopically. Mice were infected sublingually with C. albicans or C. glabrata individually, or with both species concurrently, to study their ability to cause OPC. Infection with C. glabrata alone resulted in negligible infection of tongues; however, colonization by C. glabrata was increased by co-infection or a pre-established infection with C. albicans. Furthermore, C. glabrata required C. albicans for colonization of tongues, since decreasing C. albicans burden with fluconazole also reduced C. glabrata. C. albicans hyphal wall adhesins Als1 and Als3 were important for in vitro adhesion of C. glabrata and to establish OPC. C. glabrata cell wall protein coding genes EPA8, EPA19, AWP2, AWP7, and CAGL0F00181 were implicated in mediating adhesion to C. albicans hyphae and remarkably, their expression was induced by incubation with germinated C. albicans. Thus, we found a near essential requirement for the presence of C. albicans for both initial colonization and establishment of OPC infection by C. glabrata.

Natural Sources as Innovative Solutions Against Fungal Biofilms

Fungal cells are capable of adhering to biotic and abiotic surfaces and form biofilms containing one or more microbial species that are microbial reservoirs. These biofilms may cause chronic and acute infections. Fungal biofilms related to medical devices are particularly responsible for serious infections such as candidemia. Nowadays, only a few therapeutic agents have demonstrated activities against fungal biofilms in vitro and/or in vivo. So the discovery of new anti-biofilm molecules is definitely needed. In this context, biodiversity is a large source of original active compounds including some that have already proven effective in therapies such as antimicrobial compounds (antibacterial or antifungal agents). Bioactive metabolites from natural sources, useful for developing new anti-biofilm drugs, are of interest. In this chapter, the role of molecules isolated from plants, lichens, algae, microorganisms, or from animal or human origin in inhibition and/or dispersion of fungal biofilms (especially Candida and Aspergillus biofilms) is discussed. Some essential oils, phenolic compounds, saponins, peptides and proteins and alkaloids could be of particular interest in fighting fungal biofilms.

Development and In Vivo Evaluation of a Novel Histatin-5 Bioadhesive Hydrogel Formulation against Oral Candidiasis

Oral candidiasis (OC), caused by the fungal pathogen Candida albicans, is the most common opportunistic infection in HIV(+) individuals and other immunocompromised populations. The dramatic increase in resistance to common antifungals has emphasized the importance of identifying unconventional therapeutic options. Antimicrobial peptides have emerged as promising candidates for therapeutic intervention due to their broad antimicrobial properties and lack of toxicity. Histatin-5 (Hst-5) specifically has exhibited potent anticandidal activity indicating its potential as an antifungal agent. To that end, the goal of this study was to design a biocompatible hydrogel delivery system for Hst-5 application. The bioadhesive hydroxypropyl methylcellulose (HPMC) hydrogel formulation was developed for topical oral application against OC. The new formulation was evaluated in vitro for gel viscosity, Hst-5 release rate from the gel, and killing potency and, more importantly, was tested in vivo in our mouse model of OC. The findings demonstrated a controlled sustained release of Hst-5 from the polymer and rapid killing ability. Based on viable C. albicans counts recovered from tongues of treated and untreated mice, three daily applications of the formulation beginning 1 day postinfection with C. albicans were effective in protection against development of OC. Interestingly, in some cases, Hst-5 was able to clear existing lesions as well as associated tissue inflammation. These findings were confirmed by histopathology analysis of tongue tissue. Coupled with the lack of toxicity as well as anti-inflammatory and wound-healing properties of Hst-5, the findings from this study support the progression and commercial feasibility of using this compound as a novel therapeutic agent.

Candida albicans Cek1 mitogen-activated protein kinase signaling enhances fungicidal activity of salivary histatin 5

Candida albicans is a major etiological organism for oropharyngeal candidiasis (OPC), while salivary histatin 5 (Hst 5) is a human fungicidal protein that protects the oral cavity from OPC. C. albicans senses its environment by mitogen-activated protein kinase (MAPK) activation that can also modulate the activity of some antifungal drugs, including Hst 5. We found that phosphorylation of the MAPK Cek1, induced either by N-acetylglucosamine (GlcNAc) or serum, or its constitutive activation by deletion of its phosphatase Cpp1 elevated the susceptibility of C. albicans cells to Hst 5. Cek1 phosphorylation but not hyphal formation was needed for increased Hst 5 sensitivity. Interference with the Cek1 pathway by deletion of its head sensor proteins, Msb2 and Sho1, or by addition of secreted aspartyl protease (SAP) cleavage inhibitors, such as pepstatin A, reduced Hst 5 susceptibility under Cek1-inducing conditions. Changes in fungal cell surface glycostructures also modulated Hst 5 sensitivity, and Cek1-inducing conditions resulted in a higher uptake rate of Hst 5. These results show that there is a consistent relationship between activation of Cek1 MAPK and increased Hst 5 susceptibility in C. albicans.

How does it kill?: understanding the candidacidal mechanism of salivary histatin 5

Histatins are salivary cationic peptides that provide the first line of defense against oral candidiasis caused by Candida albicans. This minireview presents a critical evaluation of our knowledge of the candidacidal mechanism of histatin 5 (Hst 5). Hst 5 is the most potent among all histatin family members with regard to its antifungal activity. The mode of action of Hst 5 has been a subject of intense debate. Unlike other classical host innate immune proteins, pore formation or membrane lysis by Hst 5 has largely been disproven, and it is now known that all targets of Hst 5 are intracellular. Hst 5 binds C. albicans cell wall proteins (Ssa1/2) and glycans and is taken up by the cells through fungal polyamine transporters in an energy-dependent manner. Once inside the fungal cells, Hst 5 may affect mitochondrial functions and cause oxidative stress; however, the ultimate cause of cell death is by volume dysregulation and ion imbalance triggered by osmotic stress. Besides these diverse targets, a novel mechanism based on the metal binding abilities of Hst 5 is discussed. Finally, translational approaches for Hst 5, based on peptide design and synergy with other known drugs, are considered a step forward for bench-to-bed application of Hst 5.

Interplay between Candida albicans and the antimicrobial peptide armory

Antimicrobial peptides (AMPs) are key elements of innate immunity, which can directly kill multiple bacterial, viral, and fungal pathogens. The medically important fungus Candida albicans colonizes different host niches as part of the normal human microbiota. Proliferation of C. albicans is regulated through a complex balance of host immune defense mechanisms and fungal responses. Expression of AMPs against pathogenic fungi is differentially regulated and initiated by interactions of a variety of fungal pathogen-associated molecular patterns (PAMPs) with pattern recognition receptors (PRRs) on human cells. Inflammatory signaling and other environmental stimuli are also essential to control fungal proliferation and to prevent parasitism. To persist in the host, C. albicans has developed a three-phase AMP evasion strategy, including secretion of peptide effectors, AMP efflux pumps, and regulation of signaling pathways. These mechanisms prevent C. albicans from the antifungal activity of the major AMP classes, including cathelicidins, histatins, and defensins leading to a basal resistance. This minireview summarizes human AMP attack and C. albicans resistance mechanisms and current developments in the use of AMPs as antifungal agents.