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 in 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 one week. 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 aw. All developmental stages of C. halotolerans showed growth after retransfer from 75 % RH. Dormant conidia survived retransfer to a medium with high aw in all cases. In addition, retransfer from 84% RH to high aw resulted in burst hyphal tips in Aspergillus and Penicillium. Cell damage of hyphae of these fungi after incubation at 75 % RH was already visible after 2 hours of incubation 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 these fungi, at a lowered steady state aw. The survival strategy of this phylloplane fungus to these dynamics in aw is discussed in relation to its morphology as studied with Cryo-SEM. Societal importance Indoor fungi cause structural and cosmetic damage in houses and are a potential threat to human health. Growth depends on water, only available at certain periods of the day (e.g cooking, showers). To know why fungi can or cannot survive indoors is important to find novel ways for prevention. Until now the ability of fungi to grow on media with little available water at a steady state (unchanging conditions) is important to evaluate whether a fungus could grow indoors. In this study, we found that the fungus Cladosporium halotolerans, a common indoor fungus, is more resistant to changes in available water than the fungi Aspergillus niger and Penicillium rubens despite the fact that the latter fungi can grow on media with low available water. Concluded, the ability of fungi to deal with changes in humidity is at least as important as the ability to grow at low water media