A comparative study of different methods to determine the total number and the survival ratio of bacteria in aerobiological samples

Estimation of the total number of microorganisms in an aerobiological sample is dependent on the handling of the sample and the analytical method utilised. In this study different microscopic methods and Coulter Counter analysis were compared. Preparation of samples for microscopy was found to be critical and the precision within samples varied considerably depending on the dilution. Improved sample handling resulted in increased precision for all the methods compared. The results showed that estimations by Coulter Counter were preferred because of its rapidity, high precision and yield. With improved sample handling and by comparing numbers of viable bacteria by plate count and of total bacteria by Coulter Counter assay, it was possible to determine the survival ratio for Escherichia coli with a coefficient of variation between samples of 13%. In addition, the use of Coulter Counter avoided underestimation of the total number which can lead to survival ratios larger than 100%.