Growth of indoor fungi under changing water conditions

Abstract

Fungi are ubiquitous in nature. Three fungal genera, Aspergillus, Cladosporium and Penicillium, have been found to be predominant in outdoor and indoor air. Fungal growth in indoor environments depends on water activity (aw) and relative humidity (RH), expressing the availability of water. In this thesis, the growth behavior of three indoor fungal species was investigated on different surfaces (agar, polycarbonate filter, and several types of gypsum) at different aw and RH. The response of these fungi to changes in RH was monitored and fungi were subjected to different techniques to study the fungal cell wall. An inventory of Cladosporium species originating from indoor air and indoor surfaces of archives, industrial factories, laboratories, and other buildings resulted in a predominance of the C. sphaerospermum species complex. Isolates from this species complex showed growth at lower aw (≥0.82) when compared to species from the C. cladosporioides and C. herbarum species complexes (≥0.85). These data indicate that xerotolerance provides advantage in colonizing indoor surfaces. The ability of developmental stages of A. niger, C. halotolerans, and P. rubens to survive changes in aw dynamics was studied. Growth of all developmental stages was halted at RH<75%, while growth continued at RH 84%. Conidia, swollen conidia, germlings, and microcolonies of C. halotolerans showed growth after retransfer from 75% RH to 0.98 aw. In contrast, only conidia of A. niger and P. rubens could reinitiate growth. Thus, C. halotolerans is more resistant to aw dynamics than A. niger and P. rubens. Gypsum is an important building material. Therefore, fungal growth on different types of gypsum was tested. Conidia of the three indoor fungi hardly germinated on natural gypsum or flue-gas gypsum, but showed outgrowth on phosphogypsum, whilst only few A. niger conidia germinated on this substrate. This could be explained by the ability of C. halotolerans and P. rubens to germinate in water, while A. niger needs organic molecules to induce germination. Melanin is hypothesized to play a role in water stress resistance in fungi. The presence of melanin can be visualized by enhancement of autofluorescence after excitation with near-infrared wavelengths. It was observed with the black wild-type conidia of A. niger as well as with conidia of the melanin biosynthesis A. niger deletion strains that formed brown (ΔbrnA), olive (ΔolvA), and fawn (ΔfwnA) conidia. Enhancement of autofluorescence was not observed with the white conidia of A. niger ΔpptA. Growth of these deletion strains of A. niger was evaluated at low aw and after changes in RH. Germination of conidia of the wild-type and deletion strains was observed at 0.81 aw, while germlings continued growth at aw≥0.83. No difference was seen between the mutant strains and the wild-type strain. Also no difference in survival of conidia and micro-colonies was seen after incubation for 1 week at RH 33-84%. The findings of this thesis give insight into the ecological adaptations of fungi to the indoor environment.