Abstract
The food- and airborne fungal genus Wallemia comprises seven xerophilic and halophilic species: W. sebi, W. mellicola, W. canadensis, W. tropicalis, W. muriae, W. hederae and W. ichthyophaga. All listed species are adapted to low water activity and can contaminate food preserved with high amounts of salt or sugar. In relation to food safety, the effect of high salt and sugar concentrations on the production of secondary metabolites by this toxigenic fungus was investigated. The secondary metabolite profiles of 30 strains of the listed species were examined using general growth media, known to support the production of secondary metabolites, supplemented with different concentrations of NaCl, glucose and MgCl2. In more than two hundred extracts approximately one hundred different compounds were detected using high-performance liquid chromatography-diode array detection (HPLC-DAD). Although the genome data analysis of W. mellicola (previously W. sebi sensu lato) and W. ichthyophaga revealed a low number of secondary metabolites clusters, a substantial number of secondary metabolites were detected at different conditions. Machine learning analysis of the obtained dataset showed that NaCl has higher influence on the production of secondary metabolites than other tested solutes. Mass spectrometric analysis of selected extracts revealed that NaCl in the medium affects the production of some compounds with substantial biological activities (wallimidione, walleminol, walleminone, UCA 1064-A and UCA 1064-B). In particular an increase in NaCl concentration from 5% to 15% in the growth media increased the production of the toxic metabolites wallimidione, walleminol and walleminone.
Highlights
Filamentous fungi inhabit a large variety of different ecological habitats [1]
Genomic analyses revealed that the ability of fungi to produce secondary metabolites has been substantially underestimated, because many of the fungal secondary metabolite biosynthesis gene clusters are not expressed under standard cultivation conditions [56]
A substantial number of secondary metabolites were detected in the cultures of Wallemia spp., only nine secondary metabolite clusters were identified in the genome of W. mellicola and eight clusters were found in the genome of W. ichthyophaga by the antiSMASH software
Summary
Filamentous fungi inhabit a large variety of different ecological habitats [1]. Competitionselected fungi are characterized by their ability to produce secondary metabolites [2, 3] and various extracellular enzymes that can be of interest to the agrochemical, food, and pharmaceutical industries [4, 5]. The detailed search for the production of secondary metabolites has been focused mainly on cosmopolitan soil borne fungi such as genera Penicillium and Aspergillus [8,9,10]. With a few exceptions such as reports on the extrolite production by halotolerant and halophilic fungi from the genera Aspergillus and Penicillium [11, 12] extremophilic fungi remained mainly overlooked, due to the general opinion that extremophiles need a smaller number of bioactive molecules to interact with a limited number of competing species in extreme environments [11, 13, 14]. The diversity of secondary metabolites in the investigated stress-tolerant or stress-selected fungi was rather low [15]. No secondary metabolites were identified for the fungus Xeromyces bisporus that is able to tolerate the lowest water activity (aw % 0.61) of all microorganisms [16]
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