EVALUATION OF NUTRIENT AGARS FOR THE ENUMERATION OF VIABLE AEROBIC HETEROTROPHS IN COOLING WATER

The agar plate method is still the method of choice for quantifying viable bacteria in water, especially in routine laboratories. Eleven nutrient agar media were compared for colony forming ability by incubating at 25 and at 30-degrees-C. Appearance of colonies was recorded against time and colony forming curves were constructed. The highest counts after 5 days (c. 75% of maximum count) were obtained on R3A agar incubation at 30-degrees-C. R3A agar at 30-degrees-C also allowed the most rapid colony development, with most of the colonies appearing between 2 and 5 days. Both plate count agar (PCA) and standard I Agar (Std I) performed poorly. PCA yielded 30% of the highest count after 5 days at 30 and at 25-degrees-C. Std I yielded 27% at 30-degrees-C and 23% at 25-degrees-C. The standard procedure of 48 h at 30-degrees-C on PCA yielded 14% of the final highest count, i.e. one log less. In contrast to PCA and StdI, R3A agar contained magnesium and pyruvate, both of which were shown to increase the plate count. R3A also had a lower nitrogen content and nitrogen to carbon ratio. The highest count after 14 days was achieved on double strength R3A (R4A) at 20-degrees-C. R4A also showed the highest colony diversity. It was therefore seen to be the most suitable for population studies. However, colony development was slower. This makes it unsuitable for plate counts, whereas R3A lets most cells develop into colonies within the first 5 days of incubation.