The use of viablue and DEFT for the detection of yeasts, moulds and bacteria

Abstract

The DEFT (Direct Epifluorescent Filter Technique) is a rapid method used for the fast detection and enumeration of food-contaminating and spoilage microorganisms. Acridine Orange was the stain originally used with this technique but there were problems with viability staining characteristics, 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 advantage over Acridine Orange. Viable and dead yeast cells are easily differentiated and this is useful in situations where the determination of dead cells is important (treatment with antibiotics, disinfectants, heat, etc.). Image analysis systems used with the stain Viablue 2 and automated counting, once perfected, should remove operator fatigue and loss of fluorescence caused by fading, since observations are made within a matter of 1–2 s. These studies have been extended to bacteria but the low intensity of fluorescence has made interpretation difficult. Whilst viable and dead cells of Bacillus cereus, Lactobacillus spp. and Staphylococcus aureus could be differentiated, smaller bacteria such as 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 can be recommended for use outside the research laboratory.