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This is a Provisional monograph.

Reasoner and Geldreich (1985) developed this special agar for heterotrophic plate counts. It was designed to recover increased numbers of bacteria from potable water samples. The medium has low nutrient concentrations and, by extending the incubation time to 14 days, significantly higher counts can be achieved compared with conventional media.

Dissolve the ingredients in the water by boiling and transfer into suitable containers. Adjust the pH to 7.2. Sterilise by autoclaving at 121°C for 15 min. The medium may be stored or cooled to 45°C and poured into suitable Petri dishes for plate counts.

A small volume (usually 1 mL) of water sample is added to the molten cooled medium and mixed. Samples are usually prepared in duplicate. Alternatively, poured and dried plates may be inoculated with sample, which is then spread using a sterile spreader.

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