Standard plate counts of drinking water: a comparison between incubation temperatures of 20 and 30 degrees C

Standard plate counts of 5085 drinking water samples gathered in the Region of Basle were carried out over a period of 9 years (1977 to 1985). Two conditions of incubation were evaluated: 20 degrees C and 30 degrees C for 72 h. In ground water samples (3048 samples) colony forming units (cfu) at 30 degrees C were found to be higher than counts at 20 degrees C incubation, 45% of the samples contained greater than or equal to 2 cfu/ml at 30 degrees versus 35% at 20 degrees C. The median was 1 cfu/ml at both temperatures. In spring water samples (2036 samples) bacterial counts at 20 degrees C were found to be higher than counts at 30 degrees C incubation, 61% of the samples contained greater than 10 cfu/ml at 20 degrees C versus 51% at 30 degrees C. The median was 19 cfu/ml at 20 degrees C incubation versus 11 cfu/ml at 30 degrees C. These differences were statistically significant with p less than 0.001 (Wilcoxon matched-pairs signed-rank test). No correlation was found between bacterial counts at 20 degrees C and bacterial counts at 30 degrees C, nor between bacterial counts and original water temperatures. It appears that incubation temperatures of 20 degrees C and 30 degrees C favor the growth of different populations of bacteria and temperature is not the only factor. However, from a practical point of view the use of only one incubation temperature seems to be justified for the purpose of judging the sanitary quality of drinking water.