A comparison of quantitative methods for measuring yeast flocculation

Two quantitative objective methods for measuring flocculation of a yeast culture were com pared and correlated with subjective estimation by eye. One method involved counting in a haemocytometer the number of free cells (i.e., cells not found in flocs) and then the total number of cells after complete deflocculation by pronase. The number of cells in flocs was derived by subtracting the number of free cells from the total number of cells. The other method made use of the decrease in turbidity of a flocculated culture on standing. Gross flocs settled to the bottom of a tube within 5 min. The decrease in turbidity was measured by a photoelectric colorimeter. The difference in turbidity readings between 0 and 5 min was assumed to represent the turbidity component due to cells in flocs. The methods were appropriately applicable to monitoring the time course of sex-directed flocculation of the fission yeast Schizosaccharomyces pombe. A culture at maximum flocculation contained 500 gross flocs ml −1 which represented up to 70% of the total number of cells. A typical primary floc occupied a volume of 1 × 10 −4 ml and contained 1 × 10 5 cells.