HiTouch™ Flexi Plate™ - R2A

Principle And Interpretation

Hitouch Flexi plates are ready to use sterile media supplied in flexible disposable plates, 55 mm in diameter. It is grid scored on the base and is irradiated to ensure perfect sterility. Medium is filled aseptically and each plate is packed in pre-sterilized plastic bag. HiTouch Flexi plate is then packed in plastic pouch wrapping. The unique flexible plate configuration ensures close contact even with uneven surfaces. These plates are specially developed for microbial testing, where count can be obtained. The Flexi plate medium formula is suitable for enumeration of treated potable water using longer incubation periods and the grids enable direct reading on the plates of the number of colonies per cm2. 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.