Bacterial diversity in the outdoor air
There is a need for baseline information about the normal abundance, distribution and composition of bacteria in the atmosphere to support many applications related to public health and international security. Such studies face significant challenges, including the broad diversity of bacteria that can be carried into the air from soil and plant sources, and the tremendous variability (both locally and regionally) in microbial load and composition owing to seasonal effects, local climate, weather patterns, local human activities, and local wind currents [27].
Bacterial community fingerprints have been used to broadly assess the composition of bacteria in urban aerosols [28] (C Kuske, unpublished). Automated ribosomal intergenic spacer analysis (ARISA) was recently used to compare bacterial composition in two aerosol samples in the Paris, France area [28], and 16S rRNA gene terminal restriction fragment length polymorphism (TRFLP) profiles were used to document the temporal, local and geographic variability of total bacterial composition in four US cities (Figure 1; C Kuske unpublished). Such community fingerprinting methods are rapid, easily replicated in spatial and temporal studies, and inexpensive (relative to the methods described below). However, they do not provide sufficient resolution to identify species when used with broadly inclusive genes like the rRNA genes, and can resolve only the more abundant types in complex samples. When calibrated using sequenced clone libraries (Figure 1), they can be useful for determining large differences in community structure. Narrowing the phylogenetic scope of the study can improve the resolution capability using community fingerprints; for example, a fingerprinting method based on heteroduplex formation was recently used to survey a large set of aerosol samples from 11 US cities for the presence of three groups of Bacillusspecies differing in relatedness to Bacillus anthracis [29 and 30].