Trehalose metabolism and stress resistance in Aspergillus niger
Abstract: Food spoilage by filamentous fungi, moulds, is a global issue of great significance. Mould growth can cause food losses due to deterioration, and adverse health effects due to toxin production. Conidia, asexual spores, are the main factors of dispersal and infection in most food-relevant moulds. They therefore constitute important objects of study when trying to improve food safety. This thesis sought to increase understanding of conidial germination and stress resistance in the common food spoilage mould Aspergillus niger, by investigating the inhibitory properties of phenyllactic acid, a lactic acid bacteria metabolite, as well as the role and metabolism of trehalose. This sugar is a major component of spores and well known for its ability to protect proteins and membranes against various kinds of abiotic stress. A thorough examination in A. niger revealed that it contains six genes involved in trehalose synthesis: tpsA-C and tppA-C. All genes were expressed during conidial germination, and conserved orthologues could be identified in related Aspergilli. However, when creating deletion mutants, only ΔtpsA, ΔtppA and ΔtppB had significantly lowered trehalose content. These three genes therefore seem to be most significant for trehalose accumulation. Degradation of internal trehalose is catalysed by trehalase; in A. niger this enzyme is encoded by treB. Studies of ΔtppA, ΔtppB and ΔtreB demonstrated that a functional synthesis as well as utilisation of trehalose is crucial for heat stress resistance of A. niger conidia. Inability to produce or degrade trehalose also negatively affects the ability of the fungus to generate conidia. Lactic acid bacteria (LAB) have long been used to ferment foods, thereby extending the shelf life. Several LAB metabolites with antifungal properties have been identified, but the underlying mechanisms of inhibition are largel unknown. We have investigated the effect of phenyllactic acid (PLA) on growth and development of single A. niger conidia. Results showed that PLA inhibits moulds in a manner similar to that of weak acid preservatives, such as benzoic acid; but it also strongly restricts asexual development at sub-inhibitory concentrations. This effect is not seen by other acids, and therefore indicates a unique mechanism of action for PLA.
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