Ethanol-induced water stress and fungal growth

Fungal growth inhibition by ethanol was compared with that caused by five other agents of water stress (at 25, 40 and 42.5°C), using Aspergillus oryzae. Ethanol, KCl, glycerol, glucose, sorbitol, and polyethylene glycol 400 were incorporated into media at concentrations corresponding to water activity (a_w) values in the range 1 to 0.75. Generally, as temperature increased there was a decrease in the a_w value at which optimum growth occurred. The a_w limit for growth on KCl, glycerol, glucose, sorbitol, or polyethylene glycol 400 media was about 0.85, regardless of temperature. However, the a_w limit for growth on ethanol media varied between 0.97 and 0.99 a_w and was temperature-dependent. Water stress accounted for up to 31, 18 and 6% of growth inhibition by ethanol at 25, 40, and 42.5°C, respectively. For media containing ethanol, the decrease in growth rate per unit of a_w reduction was greater as temperature increased. However, ethanol-induced water stress remained constant regardless of temperature, suggesting that other inhibitory effects of ethanol are closely temperature-dependent. Water stress may account for considerably more than 30% of growth inhibition by ethanol in cells that remain metabolically active at higher ethanol concentrations.