Fluorescein diacetate hydrolysis

fluorescein diacetate hydrolysis (FDA) assays can be used to measure enzyme activity produced by microbes in a sample. A bright yellow glow is produced and is strongest when enzymatic activity is greatest. This can be quantified using a spectrophotometer.

It is often used to measure activity in soil and compost samples, however 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 eucaryotic cells. Living cells will actively convert the non-fluorescent FDA into the green fluorescent compound "fluorescin", a sign of viability; while nucleus of membrane-compromised cells will fluoresce red, a sign of cell death. Currently FDA/PI staining is the standard assessment of human pancreatic islet viability with suitability for transplantation when viability score is above 70%