Microbial concentrations can be measured in terms of (i) cell concentration (the number of viable cells per unit volume of culture) or (ii) biomass concentration (dry weight of cells per unit volume of culture). The number of bacteria at a given time can be estimated by performing a total count or viable count.
In dilution method, several tubes with liquid culture media are incubated with varying dilutions of sample and the viable count is calculated from the number of tubes showing bacterial growth. This method is widely used in microbiological testing of water for presumptive coliform count in drinking water.
In the plate method, a sample is diluted and small volume of it is spread on the surface of an agar plate. The number of colonies that grow after a suitable incubation time indicates viable count of the bacteria.