Auxotrophy for uridine increases the sensitivity of Aspergillus niger to weak-acid preservatives

Weak-acid preservatives such as sorbic acid are added to foods to prevent fungal spoilage. The modes of action of weak-acid preservatives are only partially understood and, in this paper, further insight is presented into the mechanisms by which weak acids inhibit the growth of fungi. Uridine-requiring strains of Aspergillus niger were shown to be more sensitive to weak acids (including sorbic, acetic and benzoic acids) than wild-type (WT) strains. In contrast, sensitivity to other, non-acidic, antifungal substances was similar in mutant and WT strains. By complementing a pyrG(-) strain of A. niger with an intact pyrG gene, WT-like resistance to weak-acid preservatives was restored. Using 14 C-labelled uridine, sorbic acid was shown to completely inhibit uridine uptake in germinating conidia in a non-competitive manner. It is therefore proposed that the additional weak-acid sensitivity of the pyrG- strains was caused by weak-acid inhibition of uridine uptake. Several other auxotrophic strains of A. niger were screened for sensitivity to acetic, sorbic and decanoic acids. Strains auxotrophic for either adenine or uridine were found to have enhanced sensitivity but, in contrast, amino acid auxotrophs showed resistance comparable to that of the WT. Uridine auxotrophs of Saccharomyces cerevisiae were not more sensitive to weak acids compared to WT strains. In conclusion, this study describes a previously unknown mechanism of action of weak acids against the filamentous fungus A. niger, which may fundamentally affect our understanding of the preservation of food against spoilage fungi.