Tackling resistance to food preservatives: Heterogeneity in fungal spore populations

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Up to one third of all foods is spoiled by fungi. This is a major concern for global food security. Spoilage of food by fungi renders the food inedible, and potentially toxic due to formation of substances termed mycotoxins. Fungal spores are abundant in the environment and prolific contaminants, being able to develop and grow in many conditions. Despite a variety of strategies to preserve foods, spoilage microorganisms fight back and some develop resistance to particular preservatives. Soft drinks are typically acidic and these conditions tend to inhibit other organisms like bacteria. However, many species of fungi (especially the moulds) are able to grow in acidic conditions and cause spoilage. A main preservative strategy is to include weak acids like sorbic acid to inhibit growth under these conditions. At permitted levels, sorbic acid inhibits most yeasts and fungi, but a number can still grow and some can degrade the sorbic acid to non-toxic products which alter flavour. Consequently, a chronic level of mould spoilage occurs in the manufacture of soft drinks. Our work has shown that most spoilage fungi produce a small subpopulation of spores that are highly resistant to sorbic acid. Spores produced by other fungi seem to be more uniformly sensitive to the preservative. It is this ability to produce a subset of spores that are hyper-resistant, responsible for fungal growth at higher sorbic acid concentrations, which is the focus of this project. We will apply the latest genetic technologies to single spores to find out what defines these hyper-resistant spores. We will then exploit that information to develop novel methods for tracking these spores in laboratory media and beverages. Finally, we will apply these new tools to find alternative agents that can inhibit the sorbic acid-resistant spores. This approach could offer solutions for reformulating preservatives to give more complete inhibition of spoilage moulds, an area of particular interest to our industrial partners supporting this project. While soft drinks and sorbic acid resistance provide the exemplar for this work, the knowledge generated will help develop strategies for preventing fungal food spoilage more broadly.