Mechanisms of acetic acid resistance in spoilage yeasts

Currently one third of world food production is lost through spoilage. It is therefore very important to develop improved methods for the large-scale preservation of food materials. Many of the chemical preservatives approved for use in foods, beverages and pharmaceuticals are weak organic acids. These are natural compounds with a long history of apparently safe use and legislation, particularly in the EU, is unlikely to approve large-scale use of newer, more effective preservatives. A major drawback with these weak acids is that they must often be used at high level in order to inhibit growth of spoilage moulds and yeasts. Thus the resistance of spoilage yeasts to these preservatives is the major obstacle preventing a lowering of the preservative levels in many foods and beverages. We have been unravelling the weak acid preservative resistance systems of spoilage yeasts, having now established a world lead in this area. This project is focussed on understanding how spoilage yeasts become resistant to acetic acid, an understanding that will help the food industry use lower levels of acetate in preservation, thus helping to satisfy consumer demand for less-heavily preserved products, whilst maintaining microbiological safety and stability in the face of ever-tightening legislation on the use of a limited number of preservatives.It will also help the wine industry prevent acetate inhibition of the yeast during the initial stages of wine fermentations.

Our recent work is the first to apply molecular genetics to a study of organic acid food preservative resistance in spoilage yeasts. Our studies uncovered that a hitherto-unknown stress response confers resistance to the widely-used moderately-lipophilic weak organic acid preservatives. This response does not, though, confer any resistance to acetate. In recent unpublished work we have shown that the plasma membrane Fps1 aquaglyceroporin channel is the major route of acetate entry to Saccharomyces cerevisiae cells; also that important factors in this resistance are an activation of Hog1 MAP kinase and a loss of the Fps1 channel. These events appear to be interrelated, in that there is no loss of Fps1 in acetate stressed cells lacking Hog1. This project will use established molecular genetic techniques to unravel the detailed mechanisms of acetate stress sensing, leading to an activation of Hog1 MAP kinase signalling, Fps1 inactivation and the acquisition of acetate resistance by Saccharomyces cerevisiae. It will also determine if a similar system of acetate resistance applies to another major class of spoilage yeasts, the Zygosaccharomyces yeasts, determining if Fps1 also constitutes the major route of diffusional entry of acetate to the cells of Zygosaccharomyces spp.; and whether acetic acid resistance by the latter spoilage yeasts also entails inactivation of this plasma membrane channel.