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

Abstract

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 microm), 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 microm). A large portion of the clones detected at respiratory particle sizes (< 3.3 microm) 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.

Keywords: African dust; bacterial community composition; microbial transport; particle size distribution; pathogens.

Figure 1

HYSPLIT back-trajectories of air masses arriving at the sampling station during the dust storm over the eastern Mediterranean Sea. Plots show 3-day air mass back trajectories for every 6 hr between 0600 hours EEST (Eastern European Summer Time) on 24 February 2006 and 0000 hours EEST on 26 February 2006 at 3,000 m altitude (Draxler and Rolph 2003). Trajectories: 1, 24 February at 0600 hours; 2, 24 February at 1200 hours; 3, 24 February at 1800 hours; 4, 25 February at 0000 hours; 5, 25 February at 0600 hours; 6, 25 February at 1200 hours; 7, 25 February at 1800 hours; 8, 26 February at 0000 hours; 9, 26 February at 0600 hours.

Figure 2

Analysis of dust particles from the Sahara dust event by bacterial community composition and by PM₁₀ concentration at different time points. (A) Bacterial community composition in particles of different sizes. (B) PM₁₀ concentrations during the Sahara dust event. Abbreviations: Alphap., Alphaproteobacteria; Gammap, Gammaproteobacteria.

Figure 3

Rarefaction analysis of 16S rDNA sequence heterogeneity in clone libraries from the six particle size ranges collected during the Saharan dust storm in the eastern Mediterranean Sea. The total number of screened clones are plotted against unique OTUs identified by RFLP analysis. Error bars indicate SD; the diagonal line represents the 1:1 relationship where each clone is unique.