Survival of microorganisms in aerobiological samples is often assessed by a survival ratio (SR), which is the ratio between the viable or metabolic active number (MA) of microorganisms to the total number (TOT) of microorganisms. A method to determine survival ratios with flow cytometry was developed for Francisella tularensis, the causative agent of tularemia. F. tularensis is a fastidious bacteria that can be transmitted by aerosol and constitute as such a valid model organism for aerobiological studies. The total number of F. tularensis cells was determined by specific targeting with monoclonal antibodies and detected by phycoerytrine (PE) conjugated secondary antibodies. The metabolic active part of the targeted F. tularensis cells was quantified by staining with rhodamine 123 (Rh123). Application of the presented method showed higher precision compared to an earlier developed method for survival ratios, achieved with plate count (VC) and Coulter Counter (CC) measurements. The coefficient of variation between samples for the new method was below 5% for the survival ratio. Comparison of VC yield with MA yield showed consistently higher values for MA. The survival ratios of F. tularensis in samples taken before and after aerosolisation were analysed. SR for F. tularensis determined with the new method decreased approximately 19% whereas SR determined with VC and CC decreased 62% after passage through aerosol state.
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Henningson et al. (1998) studied this question.
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