Indoor fungal growth under changing water conditions

Indoor fungi cause cosmetic and structural damage in a considerable part of the European dwellings. Prolonged exposure of inhabitants to allergens and mycotoxins produced by such fungi is a potential threat to human health. A relative humidity (RH) of 80 % or higher is thought to be required for fungal growth to occur. On average the RH is below 50 % in normal buildings, suggesting a crucial role of humidity dynamics for fungal growth. Xerotolerant species of Cladosporium are predominant in the indoor environment and they thrive on various types of surfaces including glass, gypsum, wall paper, paint and wood. In order to study the fungal response to water activity (a_w) dynamics, Aspergillus niger (a_w ≥ 0.80), Cladosporium halotolerans (a_w ≥ 0.82) and Penicillium rubens (a_w ≥ 0.82) were dried in controlled humidity vessels to stop growth and were rehydrated under high a_w conditions after a week. The different developmental stages of these fungi were studied under these conditions using Cryo Scanning Electron Microscopy (cryoSEM). Growth of all developmental stages was halted during incubation below 75 % RH, while growth continued at 84 % RH. Swollen conidia, germlings, and micro-colonies of A. niger and P. rubens could not reinitiate growth when retransferred from a RH below 75 % to high a_w. All developmental stages of C. halotolerans showed growth after retransfer from 75 % RH. Dormant conidia survived retransfer to a medium with high a_w in all cases. Concluding that C. halotolerans is more resistant to a_w dynamics than A. niger and P. rubens, despite its limited growth, compared to these fungi, at a lowered steady state a_w.