Bacteria can be counted in different ways, and here are three fundamentally different methods:
- Direct counting in a so called Bürkner chamber that can be placed under a microscope. The method is relatively uncertain and is not used that often.
- Determination of CFU (Colony Forming Unit). Samples are taken from the culture is diluted and plated on agar plates, which are incubated until colonies can be observed. This method is relatively accurate, provided that dilution is done in a proper manner and the bacteria do not form aggregates in suspension cultures. Disadvantage: the answer is obtained only after approx. 24 h. See also Colony Forming Unit.
Depending upon test parameter, surface spreading or deep spreading is used in food microbiology. For surface spreading, 0.1 ml is used from the dilution series and 1.0 ml is used for for deep spreading. Thus, in deep spreading, the bacteria will be distributed in the whole volume of the "melted" agar and not only on the surface.
- Spectrophotometric determination of light scattering. The method provides a relative measure of the number of bacteria, but can be calibrated with a standard curve. See Growth Curve.