FLOW-CYTOMETRY FOR RAPID ASSESSMENT OF VIABILITY AFTER EXPOSURE TO A QUATERNARY AMMONIUM COMPOUND

The use of flow cytometry to rapidly assess the viability of Pseudomon as spp. and Staphylococcus spp. after exposure to a quaternary ammoniu m compound (QAC) was investigated using rhodamine 123 (Ph 123), Stain A (LIVE Stain) accumulating in viable but not in dead cells (Live/Dead Baclight bacterial viability kit, Molecular Probes Inc., Eugene, OR, USA), and Sytox green (Molecular Probes) accumulating in dead but not viable cells. Staining conditions were optimized for each stain. The f raction of viable cells after exposure to benzalkonium chloride was de termined by using the three staining techniques and colony counts on a gar medium. For all Staphylococcus spp. tested there was a high correl ation between the methods based on flow cytometry and colony counts ir respective of which stain was used. Although viable, all Pseudomonas s pp. tested accumulated Ph 123 poorly and about 30% failed to accumulat e LIVE stain as well. However, the correlation between colony counts a nd Sytox green labelling of Pseudomonas spp, was high. Our results ind icate that flow cytometry together with live or dead cell labelling ca n be used to study the bactericidal effect of QACs. The methods based on LIVE stain and Sytox green were simpler and less time consuming tha n Rh 123 labelling. Only Sytox green could be used with all strains of Staphylococcus and Pseudomonas tested.