Particle size distribution of airborne microorganisms and pathogens during an intense African dust event in the eastern Mediterranean

BACKGROUND: The distribution of microorganisms, and especially pathogens, over airborne particles of different sizes has been ignored to a large extent, but it could have significant implications regarding the dispersion of these microorganisms across the Planet, thus affecting human health. OBJECTIVES: We examined the microbial quality of the aerosols over the eastern Mediterranean region during an African storm to determine the size distribution of microorganisms in the air. METHODS: We used a five-stage cascade impactor for bioaerosol collection in a coastal city on the eastern Mediterranean Sea during a north African dust storm. Bacterial communities associated with aerosol particles of six different size ranges were characterized following molecular culture-independent methods, regardless of the cell culturability (analysis of 16S rRNA genes). RESULTS: All 16S rDNA clone libraries were diverse, including sequences commonly found in soil and marine ecosystems. Spore-forming bacteria such as Firmicutes dominated large particle sizes (> 3.3 mu m), whereas clones affiliated with Actinobacteria (found commonly in soil) and Bacteroidetes (widely distributed in the environment) gradually increased their abundance in aerosol particles of reduced size (< 3.3 mu m). A large portion of the clones detected at respiratory particle sizes (< 3.3 mu m) were phylogenetic neighbors to human pathogens that have been linked to several diseases. CONCLUSIONS: The presence of aerosolized bacteria in small size particles may have significant implications to human health via intercontinental transportation of pathogens.