Size distribution of viable, cultivable, airborne microbes and their relationship to particulate matter concentrations and meteorological conditions in a Mediterranean site

The biological loading of viable, cultivable airborne microbes (heterotrophic bacteria, actinobacteria and fungi) in 6 size fractions as well as the three different fractions of respirable particulate matter (PM₁, PM_2.5 and PM₁₀) and their relationship to meteorological conditions were studied in the ambient air due to health-related interests. An Andersen six stage viable particle impactor and a MAS 100 sampler were used for microbial measurements. 82 measurements were performed at three different periods (41 days) at a suburban, residential site in the city of Chania (Crete, Greece) during the period from April 2008 to June 2009. The concentrations of the viable, cultivable airborne microbes (bioaerosols) as well as of the PM₁, PM_2.5 and PM₁₀ were highly variable during the whole measurement period. Among the airborne microbes, fungi presented the most abundant taxonomic group in the ambient air. A characteristic profile of the mean size distribution of biological loading in different PM fractions was obtained for every measured microbial taxonomic group. Although, the highest concentrations of the airborne fungi and actinobacteria were determined at aerodynamic diameters between 2.1 and 3.3 μm, a nearly equal distribution of the mean concentrations of the airborne heterotrophic bacteria was observed in the six different size fractions. However, two small maxima were observed at the airborne heterotrophic bacteria distribution, one at the fraction with aerodynamic diameters between 1.1 and 2.1 μm, and at other at the coarse fraction with aerodynamic diameter larger than 7 μm. A considerable part of the airborne microbes Cycloheximide per mL of growth medium of bacteriwere resistant to drugs. Between 10 and 40 % of the viable, cultivable airborne microbes were resistant to low concentrations of drugs (5–10 μg of Streptomycin or a or fungi, respectively). Furthermore, multiple linear regression of the data showed that the variation in fungi concentrations depends on the variation in PM₁₀ mass concentration, PM₁ number concentration, relative humidity and solar radiation. Likewise, the concentration of heterotrophic bacteria was found proportional to the values of relative humidity and fungal concentration, whereas was negatively correlated to the solar radiation.