Enumerating the Viable and Total Cell Concentration of a Population

Enumerating the viable and total cell concentration of a population of a microbial isolate can be a really laborious task for a microbiologist.Especially when a microbiologist studies the same isolate for several years,it often becomes practical to determine the relationship between optical density (OD)and cell concentration.Once this relationship (determined by a standard curve)is determined,the OD of an isolate in a broth can be routinely used to determine the population's concentration.Why must a standard curve be prepared for each isolate when using OD measurements to determine cell concentration? Also describe an experiment that would generate this type of standard curve.