serial dilution technique

Serial Dilution technique and calculating CFU/ML

 Robert Koch is credited with identifying a method for bacterial enumeration, used first for the study of water quality. His article, About Detection Methods for Microorganisms in Water. was published in 1883.

INTRODUCTION: A serial dilution is the stepwise dilution of a substance in solution. It is a common practice to determine microbial counts for both liquid and solid specimens suspensions of E. coli in nutrient broth all the way to soil samples and hamburger meat. Most specimens have high enough numbers of microorganisms that the specimen has to be serially diluted to quantitate effectively.The following is a step-by-step procedure to working dilution problems, and includes some practice problems at the end.

The purpose can be determination of bacterial, fungal, or viral counts. This protocol is specific for bacterial counts (colony-forming units, CFUs), but can be modified for fungi (CFUs) and viruses (plaque-forming units, PFUs for viral counts).

THE STANDARD FORMULA: 

CFU ON AGAR PLATE:

Total dilution {USED TO MAKE PLATE FOR COLONY COUNT} X Volume plated

 To work the problem, you need 3 values—

 A colony count from the pour or spread plates.

 A dilution factor for the dilution tube from which the countable agar plate comes.

 The volume of the dilution that was plated on the agar plate.