THE USE OF VIABLUE AND DEFT FOR THE DETECTION OF YEASTS, MOLDS AND BACTERIA

The DEFT (Direct Epifluorescent Filter Technique) is a rapid method us ed for the fast detection and enumeration of food-contaminating and sp oilage microorganisms. Acridine Orange was the stain originally used w ith this technique but there were problems with viability staining cha racteristics, especially after heat treatment. The Viablue stains for the detection of viable yeasts and moulds showed good correlation when staining was compared with true viability. The stains correlated well with viability following heat treatment, and this is a major advantag e over Acridine Orange. Viable and dead yeast cells are easily differe ntiated and this is useful in situations where the determination of de ad cells is important (treatment with antibiotics, disinfectants, heat , etc.). Image analysis systems used with the stain Viablue 2 and auto mated counting, once perfected, should remove operator fatigue and los s of fluorescence caused by fading, since observations are made within a matter of 1-2 s. These studies have been extended to bacteria but t he low intensity of fluorescence has made interpretation difficult. Wh ilst viable and dead cells of Bacillus cereus, Lactobacillus spp. and Staphylococcus aureus could be differentiated, smaller bacteria such a s Escherichia coli, Listeria and Salmonella could not, and the use of microcolonies was necessary to increase fluorescence of these bacteria . The automated system still requires further development before it ca n be recommended for use outside the research laboratory.