You are here:
HETEROTROPHIC PLATE COUNT (HPC) METHODOLOGY IN THE UNITED STATES
Citation:
Reasoner*, D J. HETEROTROPHIC PLATE COUNT (HPC) METHODOLOGY IN THE UNITED STATES. Presented at NSF International Symposium on HPC Bacteria in Drinking Water - Public Health Implications, Geneva, Switzerland, 04/22-24/2002.
Description:
ABSTRACT
In the United States (U.S.), 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). The bacterial count method has evolved from the original Standard Methods (1st edition,1905) plate count which used nutrient gelatin and incubation at 20 C for 48 h., 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, replacing gelatin, allowed the use of higher incubation temperatures and resulted in the "body temperature count" (37 C) found in the 3rd through the 8th editions of Standard Methods. The change from 37 C incubation to 35 ? 0.5 C accommodated laboratories that did both milk and water analyses. By using a single temperature, fewer incubators were needed. The term "Standard Plate Count" (SPC) first appeared in 1960 (11th edition) along with plate count agar. Incubation at 20 C for the plate count was dropped from the 13th - 15th editions and few changes were made in the SPC method from the 11th edition through the 13th editions. Plate count analysis of bottled waters was included in the 14th edition (1975), calling for incubation at 35 ? 0.5 C for 72 ? 4h. Perhaps the most significant changes in plate count methods occurred with the 16th edition (1985). The term Heterotrophic Plate Count replaced the Standard Plate Count, and 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 options currently available, including low and high nutrient media, incubation temperatures ( 20 C, 28 C or 35 C), plating methods (pour plate, spread plate and membrane filter) and range of incubation times ( 24 h, 48 h, 72 h and 5 - 7 days) provide great flexibility in the application of the HPC analysis to drinking water.
__________________________
* Presented at the NSF International / World Health Organization Symposium on HPC Bacteria in Drinking Water, April 22-24, 2002, Geneva, Switzerland