Direct microscopic counting methods
Cell numbers in a suspension can be measured, except
for filamentous organisms, by direct microscopic
counts, using Petroff–Hauser or Neubauer-type counting
chambers. The former is more suitable for counting
bacteria. The counting grids, when the chamber is
covered with a glass coverslip, hold a known volume of
culture. By counting the number of cells within a
proportion of the grid, the number of cells per millilitre
can be determined. Direct microscopic counts are rapid,
but limited by their inability to distinguish living from
dead cells, unless differentiated by use of a vital staining
technique. Also, samples must contain relatively
high cell concentrations, normally a minimum of 5 ¥ 106
cells/mL.