Comparison of two methods for heterotrophic bacteria counting in drinking water for public distribution

Heterotrophic bacteria are bacteria that use organic compounds as a carbon source and thrive at ideal temperatures of 30 to 37ºC. Although its presence is not directly related to the population's health effects, its count in supply waters provides important data on the water microbiological quality, from treatment to distribution. This can be a breeding ground for more dangerous bacteria such as Legionella or E. coli, cause bad tasting water, generate corrosion or biofilm growth in pipelines. This work aimed to compare methodologies widely used for this analysis, method of membrane filtration and pour plate technique. We analyzed 59 samples from the municipal water supply system in Blumenau/SC, collected at 14 points, between May and June 2019, which 44 (74.6%) sampled at 9 points of the supply network and 15 (25.4%) sampling at 5 reservoir points, distributed throughout the city. Analyzes were performed according to the Standard Methods for the Examination of Water and Wastewater manual. The results were obtained by arithmetic mean of colony forming units (CFU) counts, using sufficient lighting and a manual colony counter with a magnifying glass. Samples with counts above 200 UFC (uncountable) were disregarded, and those with counts between 20 and 200 UFC, considered viable. All results were in accordance with the maximum number of heterotrophic bacteria allowed by Consolidation number 5, from Ministério da Saúde for drinking water. The results of pour plate analyzed had an average of 3.6 CFU/ml, and the filter membrane method an average of 23.2 CFU/ml, showing that there is a significant difference between the results (t = -4.5; df = 61 .5; p <0.01). Filter membrane methodology presented an average variation of 30 CFU/ml to duplicates (t = 5.6; df = 58; p <0.01), while pour plate did not present variation between the duplicates (t = -0.2; df = 58; p >0.01), indicating greater precision in the results. Both methodologies are easy to perform and applicable in water analysis laboratories, however, Pour Plate technique presented several advantages, such as reduced number of materials needed, less sample contact with different materials, minimizing the risks of contamination during the analysis and less fluctuation in results between duplicates.