ABSTRACT
INTRODUCTION Fungi are ubiquitous in nature and have evolved over time to colonise a wide range of ecosystems. Part of this evolution process has been the development of the ability to produce a range of extracellular chemicals known as secondary metabolites. Many fungi have been responsible for the production of very useful secondary metabolites with pharmaceutical use (e.g., penicillin, cyclosporin, the statin group) as well as those which are considered to be toxic. This group is known collectively as mycotoxins. Two of these are classed as 2B carcinogens (aflatoxin, ochratoxin). Others such as the trichothecenes, zearalenone, patulin and fumonisins are important agents in relation to contamination of human food and animal feed. Secondary metabolites may be defined as those products produced by microorganisms (and other “lower” organisms) that are not directly essential for growth (Betina, 1994). They may therefore be further defined as those metabolic products that have no known role in the “internal economy of the producer” (Williams, 1994). This contrasts with primary metabolism which may be defined as: “a summation of the interrelated enzyme catalysed reactions which are essential to growth by providing energy, synthetic intermediates and key macromolecules” (Betina, 1989). Secondary metabolites, including mycotoxins, have previously been considered to be somewhat “exotic” and poorly understood chemical substances. An understanding of this enigmatic group of chemicals is especially important in the case of the mycotoxins, be-
cause of the special threat they can pose in the human food production chain. Some studies refer to these as extrolites, of which hundreds can be produced by different spoilage fungi and which have been successfully used to aid in identification and differentiation of related species by using micro-extraction techniques (Frisvad and Filtenborg, 1983, 1989; Smedsgaard, 1997). Secondary metabolites are produced by both major groups of microorganisms – the bacteria and fungi – with the most well-known examples produced by the actinomycetes (bacteria) and the ascomycetes and deuteromycetes (fungi). Many hundreds of secondary metabolites are known, with many more undoubtedly still to be discovered. Indeed, individual microbial species may be capable of producing large numbers of metabolites, and the profile of production may change under different growth conditions, such as nutrient status and water availability. Figure 1 shows patterns of secondary metabolite production for the phyllosphere-dwelling fungus Epicoccum nigrum when grown on a range of different solid substrates. Secondary metabolites from the “higher” fungi (basidiomycetes) also exist, but are less well known because of the general difficulty of culturing this class of fungi in the laboratory. These metabolites may represent a largely untapped resource of useful chemical substances (Aldred et al., 2005).
