Agar Medium S (R2A Agar)

Principle And Interpretation

Agar Medium S (R-2 A Agar) is used for the heterotrophic plate counts and for sub culturing isolates from potable waters using longer incubation periods as per British Pharmacopoeia (1,2). It is recommended for pour plate, spread plate and membrane filter techniques. Plate count recommended for the bacterial examination of potable waters, gives an estimate of the aerobic and facultatively anaerobic bacteria, which grow best at 35°C on a rich medium (3). However these organisms may represent a small number of total bacteria as other bacteria are either unable to grow under these conditions, or grow very slowly which cannot be detected in 48 hours.

R-2 A Agar is modified for better recovery of these bacteria from treated waters under different incubation conditions (3). Many bacteria from natural waters, which contain limited nutrients at ambient temperature, grow best on the media with less nutrient levels. Moreover, they grow better at the temperatures below the routine laboratory incubation temperatures of 35 to 37°C (3). R-2 A Agar, Modified is a low nutrient medium consisting of less proteose peptone, yeast extract and glucose as compared to Standard Methods Agar. This medium allows the growth of stressed, injured and chlorine tolerant bacteria present in treated waters due to the presence of pyruvate and starch (2). The number of colonies on a plate is reported as CFU (Colony Forming Units) per volume of sample.