The indoor fungus Cladosporium halotolerans survives humidity dynamics markedly better than Aspergillus niger and Penicillium rubens despite less growth at lowered steady-state water activity

Indoor fungi cause damage in houses and are a potential threat to human health. Indoor fungal growth requires water, for which the terms water activity (a_w) and relative humidity (RH) are used. The ability of the fungi Aspergillus niger, Cladosporium halotolerans, and Penicillium rubens at different developmental stages to survive changes in a_w dynamics was studied. Fungi grown on media with high a_w were transferred to a controlled environment with low RH and incubated for 1 week. Growth of all developmental stages was halted during incubation at RHs below 75%, while growth continued at 84% RH. Swollen conidia, germlings, and microcolonies of A. niger and P. rubens could not reinitiate growth when retransferred from an RH below 75% to a medium with high a_w. All developmental stages of C. halotolerans showed growth after retransfer from 75% RH. Dormant conidia survived retransfer to medium with high a_w in all cases. In addition, retransfer from 84% RH to medium with high a_w resulted in burst hyphal tips for Aspergillus and Penicillium. Cell damage of hyphae of these fungi after incubation at 75% RH was already visible after 2 h, as observed by staining with the fluorescent dye TOTO-1. Thus, C. halotolerans is more resistant to a_w dynamics than A. niger and P. rubens, despite its limited growth compared to that of these fungi at a lowered steady-state a_w. The survival strategy of this phylloplane fungus in response to the dynamics of a_w is discussed in relation to its morphology as studied by cryo-scanning electron microscopy (cryo-SEM).