The Assessment of Antimicrobial Peptides Against Fungal Infection

Fungal infections are recurrent in the clinical environment and, annually, affect 25% of the general population worldwide, causing high morbidity and mortality rates. The indiscriminate use of broad-spectrum antibiotics, along with parenteral nutrition, permanent catheters, chemotherapy and radiotherapy, as well as immunosuppression in patients, is the most important predisposing factors for invasive fungal infections. Fungi are classified according to their morphologies, including yeasts (Cryptococcus spp.), fungi with branched hyphae (Aspergillus spp.), as well as fungi with both morphologies (yeasts and pseudohyphae, as for Candida spp.), which have all been associated with fungal infections in humans. Fungal biofilms are communities of adherent cells surrounded by an extracellular matrix. These biofilms are commonly found during infection caused by a variety of fungal pathogens. Clinically, biofilm infections can be extremely difficult to eradicate due to their resistance to antifungals and host defenses. Biofilm formation can protect fungal pathogens from many aspects of the innate immune system, including killing by neutrophils and monocytes. Currently, antifungal therapies are scarce and include only four classes of antifungal agents, polyenes, triazoles, echinocandins and flucytosine . The misuse of antifungal agents over the last two decades contributed to antifungal resistance development.Fungal resistance emergence has important clinical implications, as it limits the already small arsenal of antifungal agents, raising the idea of a "post-antifungal" era. Antimicrobial peptides (AMPs) have emerged as new biopharmaceuticals to prevent or treat fungal infections. AMPs are a class of small peptides that widely exist in nature and are an important part of the innate immune system of different organisms. AMPs have a wide range of inhibitory effects against bacteria, fungi, parasites and viruses . Defensins comprise antifungal peptides isolated from various organisms, including plants and mammals . Plant defensins are cationic and have 45-54 amino acids in length. These peptides have typically been isolated from seeds, but can also be found in other plant tissues including leaves, flowers, roots and stems. Most of the plant defensins identified so far have eight cysteine residues that favor structural stability by the formation of four disulfide bonds . In addition, structural studies have shown that plant defensins comprise a triple β-sheet with a parallel helix. Regarding their biological properties, plant defensins have shown activity against bacteria and fungi, both in their planktonic and biofilm modes of growth . Peptides from the cathelicidin family have been isolated from different species of mammals and exhibit broad-spectrum activities against fungi. Cathelicidins are characterized as cationic peptides, consisting of 12-80 amino acids that adopt an α-helix or β-sheet as secondary structures, most of which have 23-37 amino acid residues distributed in amphipathic helices, including LL-37. The activities of LL-37 have been investigated against Candida spp. strains. Human salivary histatins are a group of small histidine-rich proteins constituted from 7 to 38 amino acids first isolated from human parotid saliva. In general, histatins are a multifunctional group of proteins with antimicrobial properties that vary from broad-spectrum to moderate activities. Moreover, histatins have been reported for their effective antifungal activity