Fig. 2

Effect of maturation on heat resistance of Aspergillus niger conidia. a, Scanning electron microscopy pictures of conidiophores of A. niger taken from MEA plates (in hours or days after inoculation). Spore chain lengths were manually counted for at least 15 conidiophores per sample and scanned for the longest chain. The longest possible chain per plate was noted for each time point and highlighted in the picture. b, The maximum chain lengths found suggests a constant conidiation speed, calculated to be around ~ 80 min/conidium. Assuming conidiation speed is constant, the first conidium was formed ~ 32 h after inoculation and the first spore chain reached its maximum length ~ 57 h after inoculation. c, A representation of the maximum chain lengths found for each time point. A single spore chain takes ~ 25 h to reach maximum capacity d, HPLC analysis of 1*10⁸ conidia revealed only a fraction of the total compatible solutes are present inside young conidia when compared to matured conidia. Trehalose and mannitol concentrations show a significant increase in time (Student’s t-test p < 0.05). e, Heat treatment assay results, showing that young conidia are sensitive to heat stress. 10⁶ conidia were heat treated for 10 min at either 56 °C, 57 °C or room temperature (control) and subsequently plated confluently. A picture of the harvesting plates has been included to show the level of conidiation present on the initial plates from which the conidia were harvested to perform the heat treatment assay. f, Inactivation curves of wild type conidia harvested at different timepoints 39 h (■), 43 h (□), 48 h (▲), 64 h (◊), 72 h (♦) and 8d (●). Heat treatments were done in a water bath at 54 °C. Samples were taken for each time point, put directly on ice and subsequently serially diluted and plated. CFUs were counted after 5 days of growth. Measurements were done in biological triplicates. Linear regression lines were drawn from these curves to calculate decimal reduction values (Table 1)