Fungal spore germination, the critical stage in infection and food spoilage, and weak spot for new antifungal strategies

Foods and beverages are preserved in order to prevent both spoilage by microorganisms and to ensure food safety. Despite a variety of strategies to preserve foods, the microorganisms fight back and some develop resistance to particular preservatives. Fungi present major problems to the food and beverage industries because fungal spores are prevalent in the environment and the spores grow and develop in many conditions. While acidic conditions may prevent the growth of most bacteria, for example, many species of fungi (and especially the moulds) are able to grow in the highly acidic conditions that are common in soft drinks. Thus, other strategies must be adopted and weak acids are often added to soft drinks in order to inhibit the growth of moulds. Some species of mould are highly sensitive to weak acids but others are resistant and may degrade the weak acid to non-toxic products. One example of that is the conversion of sorbic acid to the volatile compound pentadiene, which confers a taint or off-flavour to the product. We have discovered the mechanism by which sorbic acid is converted to pentadiene and have also shown that the mould spore has that capacity, but not the fully-grown mould. Thus, we aim in this project to understand what triggers that resistance mechanism when spores grow to form the mould. From that understanding will come the means to develop better strategies to prevent food spoilage by moulds.

The hypothesis to be tested in this study is that molecular signals during the germination of fungal conidia determine the resistance of fungi to weak acid preservatives. The hypothesis has been formulated from our current and previous studies that have been funded through Defra and BBSRC Link projects. We have shown that the weak acid preservative is decarboxylated to pentadiene in germinating mould spores and that confers resistance to sorbic acid. We have identified at least some of the required genes (padA1, ohbA1 and a transcriptional regulator gene sdrA - those three genes are present as a cluster in Aspergillus niger) but also know that other genes are likely to be involved as well. We have some target genes for further study from a genome-wide transcriptomic study. We also know from physiological and biochemical studies that it is only the germinating spore that confers resistance to sorbic acid and expression of the genes identified (above) requires not only the presence of sorbic acid but also some cues from the spore development process itself, but we don't know what they are. We aim to explore this further and, thereby, provide new information on spore development (with and without sorbic acid) and to formulate new possibilities for controlling food spoilage by moulds. We will use RNA-seq rather than the Affymetrix gene chips that we have used to date because of the greater depth of information that can be gained. The group has used RNA-seq in other studies. We will also extend our studies away from the model spoilage mould A. niger to related species of moulds that we have isolated from spoiled beverages, and which have either low (Trichoderma viride) or high (Trichoderma atroviride) resistance to sorbic acid.

Who will benefit from this research?

Academics will benefit from this research, as outlined within the Academic Beneficiaries section. Furthermore, several companies will benefit and that is confirmed by the contributions made by companies as part of this Industrial Partnership Award. The two companies who are offering to contribute funds have a focus in different commercial activities. DSM is a large and diverse organisation with interests in the supply of ingredients for food production that includes applications in food preservation. GSK is also a large and diverse company and this project is specifically relevant to their production of soft drinks (e.g. Ribena, Lucozade). One aim of this proposal is to apply the new knowledge to improve anti-fungal food preservation practices and, if that is achieved, the consumers will benefit, as will the producing companies that currently have to deal with complaints and take remedial action.

How will they benefit from this research?

This research will contribute to improving the nation's health and wealth. Fungal spores are the starting point to contamination of crops, humans and other animals, foods and beverages. The outgrowth of fungal spores is the critical stage in the production of mycelia and infection of crops and humans. Thus, more information on that critical stage in the fungal lifestyle will contribute knowledge that will help in combating disease and in the spoilage of crops and foods/beverages. The proposed research will explore the fundamentals of spore outgrowth (with beneficiaries in all the areas mentioned) but also in relation to the specific use of weak acid preservatives in foods/beverages. In the latter area, the beneficiaries will be both consumers and producers of soft drinks.