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 (aw) 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 aw dynamics was studied. Fungi grown on media with high aw 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 aw. All developmental stages of C. halotolerans showed growth after retransfer from 75% RH. Dormant conidia survived retransfer to medium with high aw in all cases. In addition, retransfer from 84% RH to medium with high aw 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 aw dynamics than A. niger and P. rubens, despite its limited growth compared to that of these fungi at a lowered steady-state aw. The survival strategy of this phylloplane fungus in response to the dynamics of aw is discussed in relation to its morphology as studied by cryo-scanning electron microscopy (cryo-SEM).