Direct Count Methods involve actually counting the cells in a population. In a direct microscope count, a
very small sample (such as 10microL of a cell suspension) of the population is placed into a special
microscope slide that contains a very small counting chamber that has a known volume. The number of
bacterial cells visible in the chamber is counted. Because the chamber has a specific volume, the
concentration (cells/volume) of bacteria in the population is known. Bacterial cells can also be counted
electronically in machines called Coulter counters. In this method, a sample from the population is
placed in the machine. The sample is passed between two electrodes. Every time a cell passes between
the electrodes it causes a disturbance in the electrical field, and the cell is counted. Both direct
microscope counts and electronic cell counts have the advantage that the population count is
determined immediately. However, they both have the disadvantage in that they do not distinguish
between living and dead cells.