Directions:
R2A Agar is a ready to use solid media in glass bottle. The medium is pre-sterilized; hence it does not need sterilization.
Medium in the bottle can be melted either by using a pre-heated water bath or any other method. Slightly loosen the cap before
melting. When complete melting of medium is observed dispense the medium as desired and allowed to solidify.
Principle And Interpretation:
The heterotrophic plate count (HPC), formerly known as the standard plate count is a procedure for estimating the number
of live heterotrophic bacteria in water and measuring changes during water treatment, in distribution systems or in swimming
pools. R-2A Agar is recommended by APHA (1, 2) for estimating the heterotrophic plate count by the pour plate, spread plate
or membrane filter procedure. R-2A Agar is formulated as per Reasoner and Geldreich (3). Stressed or injured organisms
during water treatment are unable to grow on high nutrient media, since the faster growing organisms outgrow the former
(4). Therefore the use of a low nutrient medium like R-2A Agar incubated for longer incubation periods allows these stressed
organisms to grow well.
Many bacteria from natural waters which contain limited nutrients at ambient temperature, grow best on the media with less
nutrient levels. They grow better at the temperatures below the routine laboratory incubation temperatures of 35 to 37°C (4).
Casein acid hydrolysate, proteose peptone and yeast extract provide nitrogen, vitamins, amino acids, carbon and minerals.
Dextrose serves as an energy source. Soluble starch aids in the recovery of injured organisms by absorbing toxic metabolic
byproducts while sodium pyruvate increases the recovery of stressed cells. Magnesium sulphate is a source of divalent cations
and sulphate. Dipotassium phosphate is used to balance the pH of the medium. The number of colonies on a plate are reported
as CFU (Colony Forming Units) per volume of sample.
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