Among others, ATP is a general indicator for the presence of living cells. ATP can be measured in a very sensitive way, using firefly extracted from Photinus pyralis. The light emission is in the range between 500 to 700 nm wavelength  and the assay requires the presence of the luciferase, luciferin, magnesium and oxygen (Figure 1). The measured amount of light is proportional to the ATP in the sample. In optimum conditions 1 photon of light is produced by 1 molecule of ATP .
ATP as an Indicator of Microbiological Activity in Tap Water
The DWD  requires from Member States to take all measures to ensure that water intended for human consumption is free from any micro-organisms, parasites and from any substances, which in numbers or concentrations, constitute a potential danger to human health. Traditional methods to evaluate the presence of bacteria in drinking water samples, such as Heterotrophic Plate Count (HPC), need a few days of incubation and require selection of appropriate temperature and medium. Another important disadvantage is that a small fraction of microorganisms is able to cultivate on artificial mediums .