The simultaneous measurement of protein and DNA content of yeast on a cell by cell basis is described. A problem associated with partially overlapping fluorescence emission bands of the two fluorochromes is discussed. The rapid flow cytometric assay will be useful to monitor cell growth in industrial fermentation processes.

For economic reasons, the brewing industry is extremely interested in methods leading to the rapid discovery and identification of unwanted microorganisms. Immunofluorescence and flow cytometry are offered as a solution in the task, to detect and count contaminating microorganisms in yeasts.

The practical use of flow cytometry is shown in several microbial assays. Recent technical improvements in the optics and electronics of flow cytometric systems as well as in staining techniques permit the measurements of minute cellular components such as the cellular DNA and the protein content of bacteria, algae, moulds and yeasts. Single cell ingredients can be measured by this assay according to their specific stainability. The cell DNA was stained by propidium iodide while the cell protein was fluorochromed by fluorescein-iso-thiocyanate. The DNA synthesis of Saccharomyces cerevisiae and Saccharomyces pastorianus runs discontinuously while the protein content increases continuously during the vegetative growth. The different stages of DNA synthesis of yeast cells can be divided into two 'gap' phases, a synthesis and a mitosis period, corresponding to Howard and Pelc's model of DNA synthesis. Living and dead cells can be counted differentially after staining with Erythrosine B. The red fluorescence of the chlorophyll in algae can readily be used to determine the chlorophyll content of these cells.

Changes in protein content during the cell cyle of microorganisms can be monitored rapidly by flow cytometry.

Flow cytometry is a new method to measure fluorescing cells in liquid media in a few minutes. This rapid technique in combination with immunofluorescence can be employed to determine the purity of yeast cultures. The immunofluorescence technique is based on fluorescent labelling of cells using an antiserum raised in rabbits. Non-fluorescent cells and immunological stained cells are counted separately.

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