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A THUMBNAIL HISTORY OF HETEROTROPHIC PLATE COUNT (HPC) METHODOLOGY IN THE UNITED STATES
Reasoner*, D J. A THUMBNAIL HISTORY OF HETEROTROPHIC PLATE COUNT (HPC) METHODOLOGY IN THE UNITED STATES. Presented at NSF International Symposium on HPC Bacteria in Drinking Water, Geneva, Switzerland, 4/22-24/2002.
Over the past 100 years, the method of determining the number of bacteria in water, foods or other materials has been termed variously as: bacterial plate count, total plate count, total viable plate count, aerobic plate count, standard plate cound and more recently, heterotrophic plate count. In the US the history of bacterial plate counting methods used for water can be traced largely through "Standard Methods for the Examination of Water and Wastewater Standard Methods", currently published jointly by the American Public Health Association, the American Water Works Association and the Water Environment Federation. The bacterial count method has evolved from the original "Standard Methods" (1st edition, 1905) plate count which used nutrient gelatin and incubation at 20C for 48 hrs, to the HPC method options in the latest edition of "Standard Methods" that provide greater flexibility of application, depending on the data needs of the water analyst. The use of agar-agar as a gelling agent to replace gelatin (liquifies at 28C) allowed the use of higher incubation temperatures and resulted in "body temperature count" (37C) found in the 3rd edition (1917) through the 8th edition (1936). A change from 37C incubation to 35 +/- 0.5C was made to accommodate laboratories that did both milk and water analyses; by using a single temperature, fewer incubators were needed. The use of the term "Standard Plate Count" first appeared in 1960 (11th edition) along with plate count agar. The option of incubation at 20C for the plate count dropped from the 13-15 editions, and few changes were made in the SPC method from the 11th through the 13th editions. HPC analysis of bottled waters was included in the 14th edition (1975). Perhaps the most significant changes in plate count methods occurred with the 16th edition. The term Heterotrophic Plate Count replaced the Standard Plate Count. The spread plate and membrane filter methods were added along with new media for pour and spread plates (R2A agar and NWRI agar, both low-nutrient) and for the membrane filter method (mHPC medium). The use of low nutrient media, lower incubation temperature, and longer incubation times, results in higher plate count results for most water samples. The HPC method options adopted in the 16th edition have been carried through to the most recent edition, the 20th. The options currently available, including low and high nutrient media, incubation temperature, plating methods and range of incubation times provide great flexibility in the application of the HPC determination.