A method for multiple synchronous collection of airborne organisms and the effects on colony counts of various processing procedures

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Abstract
A method is described for synchronous collection on agar of 10 similar specimens of airborne bacterial colony-forming units (cfu) for comparative experiments. The system delivers 50 to 100 cfu per specimen with a coefficient of variance of 13 among the 10 specimens. After collection, different methods for removing colonies from the agar surface and counting them were employed. A progressive increase in colony counts was noted when increasingly destructive procedures were used. The increases noted were 4% by wetting, 30% by jet lavage, 58% by pulsed jet lavage, 82% by blending, 130% by spreading and 340% by grinding. As airborne cfu consist mainly of skin squames with multiple organisms attached, disruption of cfu is proposed as the cause of the increases. Membrane filtration of wash fluid containing cfu from the air resulted in a 47% decrease in colony counts when compared with pour-plating. Destructive processing techniques also resulted in increased variability in colony counts. The break up of occasional exceptionally large cfu is a probable explanation. The procedure described is suitable for investigating the behaviour of airborne micro-organisms and can be modified to model surgical wound contamination by replacing the agar with tissue.