Direct Methods

Viable plate count

Membrane filtration

Petroff-Hauser cell count (cytometer)

Coulter Counter - counts cells as they disrupt an electric current flowing across a detector.

Flow Cytometer - counts cells passing across a light detector.  Can be used with fluorescent antibodies.

Most Probable Number (MPN) - uses lactose broths.  After incubation, a 3-digit number is produced based on the number of positive tubes per set.  The number corresponds to a MPN/100 ml of water on a statistical table.

Indirect Methods

Metabolic activity - using measurements of waste production and nutrient depletion to determine population growth.

Dry weight (biomass) - organisms in broth are dried and their biomass measured.