Fluorescein diacetate (FDA) hydrolysis assays can be used to measure the enzyme activity of microbes in a sample.
A bright yellow-green glow is produced and is strongest when enzymatic activity is greatest.
FDA hydrolysis is often used to measure activity in soil and compost samples; however, it may not give an accurate reading if microbes with lower activity phases, such as esterases, cleave the fluorescein first.
It is also used in combination with propidium iodide (PI) to determine viability in eukaryotic cells.
FDA stock solution is prepared by dissolving 5 mg of fluorescein diacetate in 1 ml acetone,[1] and sucrose may be added for live cell viability testing.