Effects of Water Activity on the Radial Growth of Aspergillus niger on Solid Medium

The effects of water activity (aw) on the radial growth of Aspergillus niger on Malt Extract Agar was investigated using sodium chloride and glycerol mixtures. Varied concentrations of each solute was used to modify the water availability of the medium. The Petri plates were incubated at 31 ± 2°C representative of the Ghanaian ambient temperature. The inhibitory effects showed by two solutes on the growth of the A. niger was qualitatively similar but showed quantitative differences. The growth rate was erratic with no clear cut generalized response. On the whole, growth was better on glycerol-modified medium compared with the growth on the sodium chloride modified medium. Growth was optimal at the range of 0.904-0.947aw on the glycerol-modified medium and 0.800-0.821aw on the sodium chloride medium. Growth was severely retarded at 0.768aw when the water availability was adjusted with seventy-five grams (75 g) sodium chloride. Glycerol used to modify water availability of a medium may serve as carbon and energy sources to promote growth of fungi. On the other hand, high concentration of sodium chloride as solute to control water activity of a medium may be toxic to the organism hence the differences in the growth pattern observed. Microorganisms react not only to water activity but also the solute adjusting the water availability.