Enzymatic activity measurement detects specific enzymes produced by target organisms directly in the water sample. For E. coli, the enzyme β-D-Glucuronidase (GUS) is highly specific — it is produced almost exclusively by E. coli cells. A fluorogenic substrate is added to the sample; if E. coli is present, the enzyme cleaves the substrate, producing a measurable fluorescence signal within minutes. The signal is proportional to the number of active (living) cells.