Automated measurement and quantification of heterotrophic bacteria in water samples based on the MPN method

Quantification of heterotrophic bacteria is a widely used measure for water analysis. Especially in terms of drinking water analysis, testing for microorganisms is strictly regulated by the European Drinking Water Directive, including quality criteria and detection limits. The quantification procedure presented in this study is based on the most probable number (MPN) method, which was adapted to comply with the need for a quick and easy screening tool for different kinds of water samples as well as varying microbial loads. Replacing tubes with 24-well titer plates for cultivation of bacteria drastically reduces the amount of culture media and also simplifies incubation. Automated photometric measurement of turbidity instead of visual evaluation of bacterial growth avoids misinterpretation by operators. Definition of a threshold ensures definite and user-independent determination of microbial growth. Calculation of the MPN itself is done using a program provided by the US Food and Drug Administration (FDA). For evaluation of the method, real water samples of different origins as well as pure cultures of bacteria were analyzed in parallel with the conventional plating methods. Thus, the procedure described requires less preparation time, reduces costs and ensures both stable and reliable results for water samples.