Antifungal Efficacy of Redox-Active Natamycin Against Some Foodborne Fungi—Comparison with Aspergillus Fumigatus
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foods Article Antifungal Efficacy of Redox-Active Natamycin against Some Foodborne Fungi—Comparison with Aspergillus fumigatus Jong H. Kim 1,* , Christina C. Tam 1 , Kathleen L. Chan 1, Luisa W. Cheng 1, Kirkwood M. Land 2, Mendel Friedman 3 and Perng-Kuang Chang 4 1 Foodborne Toxin Detection and Prevention Research Unit, Western Regional Research Center, USDA-ARS, 800 Buchanan St., Albany, CA 94710, USA; [email protected] (C.C.T.); [email protected] (K.L.C.); [email protected] (L.W.C.) 2 Department of Biological Sciences, University of the Pacific, 3601 Pacific Avenue, Stockton, CA 95211, USA; kland@pacific.edu 3 Healthy Processed Foods Research Unit, Western Regional Research Center, USDA-ARS, 800 Buchanan St., Albany, CA 94710, USA; [email protected] 4 Food and Feed Safety Research Unit, Southern Regional Research Center, USDA-ARS, 1100 Robert E. Lee Boulevard, New Orleans, LA 70124, USA; [email protected] * Correspondence: [email protected]; Tel.: +1-510-559-5841 Abstract: The fungal antioxidant system is one of the targets of the redox-active polyene antifungal drugs, including amphotericin B (AMB), nystatin (NYS), and natamycin (NAT). Besides medical applications, NAT has been used in industry for preserving foods and crops. In this study, we inves- tigated two parameters (pH and food ingredients) affecting NAT efficacy. In the human pathogen, Aspergillus fumigatus, NAT (2 to 16 µg mL−1) exerted higher activity at pH 5.6 than at pH 3.5 on Citation: Kim, J.H.; Tam, C.C.; a defined medium. In contrast, NAT exhibited higher activity at pH 3.5 than at pH 5.6 against Chan, K.L.; Cheng, L.W.; Land, K.M.; foodborne fungal contaminants, Aspergillus flavus, Aspergillus parasiticus, and Penicillium expansum, Friedman, M.; Chang, P.-K. with P. expansum being the most sensitive. In commercial food matrices (10 organic fruit juices), food Antifungal Efficacy of Redox-Active ingredients differentially affected NAT antifungal efficacy. Noteworthily, NAT overcame tolerance of Natamycin against Some Foodborne the A. fumigatus signaling mutants to the fungicide fludioxonil and exerted antifungal synergism Fungi—Comparison with Aspergillus with the secondary metabolite, kojic acid (KA). Altogether, NAT exhibited better antifungal activity fumigatus. Foods 2021, 10, 2073. at acidic pH against foodborne fungi; however, the ingredients from commercial food matrices https://doi.org/10.3390/ presented greater impact on NAT efficacy compared to pH values. Comprehensive determination foods10092073 of parameters affecting NAT efficacy and improved food formulation will promote sustainable Academic Editor: Luís Abrunhosa food/crop production, food safety, and public health. Received: 5 July 2021 Keywords: antifungal; natamycin; oxidative stress; pH; polyenes; redox-active; drug resistance Accepted: 27 August 2021 Published: 2 September 2021 Publisher’s Note: MDPI stays neutral 1. Introduction with regard to jurisdictional claims in One Health recognizes that people, animals, plants, and their shared environment are published maps and institutional affil- highly interconnected. While recently One Health has focused the interface between human iations. and animals, especially emerging infectious diseases, the scope of One Health also includes the prevention of foodborne diseases and contamination, noninfectious diseases, and environmental (agricultural) and ecosystem health [1]. For instance, invasive aspergillosis (IA) is a life-threatening infection caused by filamentous fungi in the genus Aspergillus. The Copyright: © 2021 by the authors. most prevalent causative agent for IA is Aspergillus fumigatus, followed by Aspergillus flavus, Licensee MDPI, Basel, Switzerland. Aspergillus niger, and Aspergillus terreus [2,3]. Immunocompromised people or patients are This article is an open access article at a higher risk of developing fungal diseases [4]. Notably, besides human–animal interface, distributed under the terms and invasive aspergilli infection could also be acquired from contaminated foods (A. fumigatus) conditions of the Creative Commons or crops (A. flavus), indicating human infections, such as IA, also involves food safety Attribution (CC BY) license (https:// and environmental (agricultural) health issues [5]. However, control of fungal diseases is creativecommons.org/licenses/by/ increasingly problematic due to the limited number of effective drugs/fungicides available 4.0/). Foods 2021, 10, 2073. https://doi.org/10.3390/foods10092073 https://www.mdpi.com/journal/foods Foods 2021, 10, 2073 2 of 15 for antifungal intervention [6]. Especially, cross-resistance of fungi to azole and polyene drugs has become a prominent public and environmental health issue [7]. Azoles, such as voriconazole (VRZ), itraconazole (ITR), or posaconazole (POS), are broadly used for treating human fungal pathogens. The primary mechanism of antifungal action of azoles is to inhibit the lanosterol 14-α sterol demethylase (CYP51 of the cytochrome P450 monooxygenase (CYP) superfamily) involved in fungal ergosterol biosynthesis [8,9]. Polyene drugs, such as amphotericin B (AMB), nystatin (NYS), and natamycin (NAT) (also known as pimaricin), contain conjugated double bonds in the structure, providing high affinity to ergosterol moiety of membranes in most fungal species [10]. Sequential combination antifungal therapy is sometimes necessary for effective control of fungi [7]. However, as observed in the yeast pathogens [11], prior exposure of A. fumiga- tus to azoles has been shown to lower the susceptibility of the fungal biofilms or germlings to polyenes such as AMB [12]. Of note, A. fumigatus pre-exposure to VRZ activated the cellular stress defense (namely adaptive resistance mechanisms) via the heat-shock protein 90 (Hsp90), which subsequently conferred AMB resistance, whereas pharmacological inhi- bition of Hsp90 by geldanamycin significantly enhanced (4- to 8-fold) the AMB susceptibil- ity of fungal biofilm [12]. Hsp90 is a chaperone of calcineurin, a conserved Ca2+-calmodulin (CaM) activated protein phosphatase 2B, involved in the calcium-dependent signaling pathway for fungal virulence and stress response [13]. Therefore, studies indicated that azole-potentiated fungal tolerance to AMB is exerted via Hsp90-mediated oxidative stress defense [14]. We also determined previously that the cellular antioxidant system was a viable target in the antifungal action of azoles or AMB, where the mitochondrial superoxide dismutase (Mn-SOD), mitogen-activated protein kinases (MAPKs), etc., play crucial roles in fungal defense against azole- or AMB-induced oxidative stress [15,16]. Azoles are also increasingly applied in agricultural industry to control phytopathogenic fungi [17]. However, an azole-resistant A. flavus has been identified, where the T788G mutation in the cyp51C gene conferred the azole resistance [18]. A. flavus contaminates various agricultural commodities (during pre- and post-harvest) by producing the car- cinogenic mycotoxins aflatoxins (AFs) [19]. Mutants of the other AF-producer Aspergillus parasiticus, having the G54W mutation in the cyp51A gene, produced AFs at concentra- tions significantly higher (up to 3-fold) than the wild-type strain [20]. Currently, more than 25% of total fungicide sales are azoles such as propiconazole or tebuconazole [17]. Therefore, once incorporated into the food supply chain, the azole-treated fungi, such as AF-overproducing cyp51 mutants, will not only debilitate public food safety, but also cause significant economic losses, especially when the fungi are exposed subsequently to polyene fungicides during food production. Among polyene molecules, NAT has been used widely for control of agricultural fungi infecting various crops (mushroom, tomato, strawberry, etc.) or contaminating pro- cessed foods (table olives, fruit juices, cheeses, yogurt, wines, etc.) (Figure1; Table S1). For example, NAT was applied for postharvest disease control against Botrytis cinerea (to protect mandarin fruit), especially for managing isolates resistant to fungicides with different modes of action [21]. NAT also effectively controlled quinone outside inhibitor (QoI)-resistant Colletotrichum acutatum triggering strawberry crown rot, where the disease severity and plant mortality in field were reduced while fruit yield was significantly in- creased [22]. NAT also exhibited a synergistic antifungal activity with other compounds such as ferulic acid (FA), where coapplication of NAT and FA effectively inhibited the growth of Aspergillus japonicus and Gilbertella persicaria contaminating blackberry fruits [23]. Meanwhile, NAT has also exerted anti-mycotoxigenic capability; NAT effectively dis- rupted ochratoxin production by strains of Aspergillus carbonarius in grape juice-based medium [24]. In the baseline estimation of the postharvest pathogens, Penicillium sp., mean effective NAT concentrations for inhibiting 50% mycelial growth were determined as 1.54 and 1.14 µg mL−1 for Penicillium digitatum and Penicillium expansum, respectively [25]. Noteworthily, organic acids, such as lactic or citric acids, functioned as sensitizing agents to NAT in the green mold, P. digitatum, which resulted in a significant reduction in green Foods 2021, 10, 2073 3 of 15 mold contamination in lemon fruit [26]. In the food spoilage fungus, Penicillium discolor, NAT exhibited a different mode of action than other polyenes such as NYS; while NYS disrupted fungal plasma membrane, NAT could not permeabilize germinating conidia but inhibited their endocytosis [27]. Figure