The Great Indoors

Indoor environments, where the average person in an industrial nation spends ~90% of his or her life, represent the most important interface between humans and microbes. Examples of well-known fungi include a few human pathogens, allergens, agents of structural rot and food spoilers. Indoor fungi’s prominent role in successful litigation around the world contributes to rising costs for various industries and insurance companies. Increasingly strict standards for indoor sanitation have resulted in regulatory agencies and private industries seeking to quantify building health. Mould surveys that target the relatively few visibly apparent fungal species or those readily cultivable on artificial media are now standard, and a U.S. Environmental Protection Agency-developed set of real-time PCR probes facilitates their quantification. However, the recent rise in fungal infections caused by species formerly considered benign but now seen as causing disease in immunosuppressed humans, and a vastly increased resolution of indoor fungal composition afforded by culture-independent sampling methods, force us to reconsider what constitutes a normal indoor environment and the factors that shape it. Recent efforts to describe the processes shaping indoor and urban fungal composition show temporal effects and modest correlations with human activity. The existence of global-scale patterns in fungal composition, however, is unexamined, despite evidence of biogeographical patterning in other microbial systems.

As with larger organisms, bacterial and archaeal composition is determined by both the contemporary environment and historical processes such as dispersal. The relative importance of these factors and the particular environmental variables involved depend on the taxa and habitat sampled. For indoor fungi whose association with highly mobile humans presents opportunities for global dispersal, it was presumed that most taxa would be relatively cosmopolitan on a global scale, and that the local indoor environment (as determined by building function, construction material, or circulation system) would play a relatively large role in shaping composition. As a result of the combination of presumably high dispersal rates between indoor habitats and the highly selective indoor environment, little influence of the outdoor environment was expected on fungal composition. But are these assumptions true?


Indoor fungal composition is geographically patterned and more diverse in temperate zones than in the tropics. PNAS USA June 28 2010 doi: 10.1073/pnas.100045410
Fungi are ubiquitous components of indoor human environments, where most contact between humans and microbes occurs. The majority of these organisms apparently play a neutral role, but some are detrimental to human lifestyles and health. Recent studies that used culture-independent sampling methods demonstrated a high diversity of indoor fungi distinct from that of outdoor environments. Others have shown temporal fluctuations of fungal assemblages in human environments and modest correlations with human activity, but global-scale patterns have not been examined, despite the manifest significance of biogeography in other microbial systems. Here we present a global survey of fungi from indoor environments using both taxonomic and phylogeny-informative molecular markers to determine whether global or local indoor factors determine indoor fungal composition. Contrary to common ecological patterns, we show that fungal diversity is significantly higher in temperate zones than in the tropics, with distance from the equator being the best predictor of phylogenetic community similarity. Fungal composition is significantly auto-correlated at the national and hemispheric spatial scales. Remarkably, building function has no significant effect on indoor fungal composition, despite stark contrasts between architecture and materials of some buildings in close proximity. Distribution of individual taxa is significantly range- and latitude-limited compared with a null model of randomized distribution. Our results suggest that factors driving fungal composition are primarily global rather than mediated by building design or function.


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