Detection of airborne microbes in a composting facility by cultivation based and cultivation-independent methods
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Federal Institut for Work Safety and Health, Berlin, Germany
Institute for Applied Microbiology, University of Giessen, Giessen, Germany
Udo Jäckel   

Federal Institut for Work Safety and Health, Nöldnerstr. 40-42, 10317 Berlin, Germany
Ann Agric Environ Med. 2007;14(1):81–85
Standard methods for quantification of airborne bacteria are based on cultivation and counting of grown colonies. From complex natural environments it is known that only a small fraction of the total number of cells can be cultivated on routinely used agar-media. Direct microscopic cell counting after DNA-staining usually generates higher concentrations of one to two magnitudes. The objective of the presented study was to compare the concentrations of airborne bacteria sampled in a composting facility by using for any sample the cultivation on trytic soy agar (TSA) – agar, as well as direct counting after DAPI-staining. The concentrations after counting grown colonies were within a range of 105-107 cfu m-3. Concentrations of direct counted cells ranged between 106-109 microbes m-3. In these comparative measurements only 1.5-15.3% of the airborne bacterial cells enumerated by direct counting formed countable colonies after incubation on TSA-agar. Obviously, cultivation based methods underestimate the real amount of airborne microbes. In addition, from literature it is known that inactive or even dead cells can also have the potential to cause health effects. Consequently, a risk assessment based only on measuring colony forming units may, in some cases, not be sufficient.