Colony counts in drinking water bacteriology--importance of media and methods.

The productivity of three different plate count media were compared on unchlorinated drinking water samples. The R2A medium gave constantly higher counts than both Kings agar B and PCA, when incubated at 21 degrees C for 3 to 14 days. Spread plates were superior to pour plates for both R2A and Kings agar B, but for the R2A medium this difference diminished with time of incubation. It is suggested that this could partly be assigned to its content of sodium pyruvate as a H2O2 scavenger. Experiments with different strengths of the R2A medium showed that not only its complexity but also the low concentration of nutrients supplied is of significant importance to the productivity of the original formulation of this medium.