Detection and quantification of viable airborne bacteria and fungi using solid-phase cytometry

Abstract

This protocol describes the use of solid-phase cytometry for the enumeration of airborne bacteria and fungi. In contrast with conventional methods, accurate results can be obtained in real time, especially for air samples with low numbers of microorganisms. Air samples are collected by impaction on a water-soluble polymer that is subsequently dissolved. Part of the sample can be filtered over two membrane filters with different pore sizes. One filter is used to obtain a total count of all viable microorganisms, and a second filter is used to determine the number of airborne fungi. Microorganisms present on the filter are labeled with a viability substrate and subsequently detected and quantified using a solid-phase cytometer. The detected spots are microscopically validated using an epifluorescence microscope to discriminate between bacteria, fungi and fluorescent particles. The whole procedure takes 5 h to complete and results in the accurate quantification of airborne bacteria and fungi for samples with a low or high microbial load.