A filtration method for the determination of concentration of microorganisms in air.

Certain important industrial fermentations require substantial amounts of sterile air that are commonly obtained by filtration through beds of fibers, granular solids, or specially prepared filter media (Gaden and Humphrey, 1956). To control the efficiency of such sterilization filters, several methods to estimate the microorganism content of the air have been proposed. These methods can be classified into three fundamental types: 1. The air is passed through a porous material, such as the Millipore filter' (Humphrey and Gaden, 1955), that retains the microorganisms; the filter is then disaggregated in sterile water and the suspension is plated for counting. This method applies only when the air stream is very slow. 2. The air stream is directed to a solid culture medium (Bourdillon, 1941) that retains some of the microorganisms on its surface. This method is inadequate for longtime experiments because the culture medium dehydrates and shrinks. It also needs carefully controlled experimental conditions to assure good results. 3. The air is bubbled through a liquid with specially designed impingers (Rosebury, 1947). The suspension of microorganisms obtained is then plated for counting. This method also applies when the air stream is slow. This report describes a simple filtration method for the estimation of the concentration of microorganisms in the air. Air is passed through a filter paper mounted in a proper device; the microorganisms retained are counted on the same filter paper conveniently moistened with a suitable culture medium. This method can be used with air streams of relatively high rates, and eliminates the counting of microorganisms by plating.