Adaptation of Aspergillus niger to acidic conditions and its relationship to salt stress and miconazole

Adaptation of Aspergillus niger to acidic conditions was evaluated quantitatively at a single hypha level by a continuous measurement system. Hyphal growth was inhibited by exposure to acidic conditions (pH 2.4 or lower). Hyphae resumed extension growth after 30 min sustained exposure to pH 2.4 and 2.2, but did not recover from exposure to pH 2.0. However, hyphal growth was maintained at even lower pH values if the decrease in medium pH was carried out in a stepwise fashion. In this way, hyphal growth continued until pH was reduced to 1.4. This hyphal adaptation occurred within 20 min of exposure to acidic conditions, but was lost during a subsequent 30-60 min wash with unsupplemented medium (pH 5.1). Hyphae adapted to acidic conditions were insensitive to the antifungal agent miconazole, at concentrations inhibitory to unadapted cells. By contrast, hyphal sensitivity to miconazole was unaffected by prior adaptation to salt stress, and adaptation to acidic conditions had no effect on the sensitivity to salt stress.