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.

Turbidity