Potential problems with fluorescein diacetate assays of cell viability when testing natural products for antimicrobial activity

There are two potential problems in the use of fluorescein diacetate (FDA) as a measure of cell viability. The first is the hydrolysis of FDA to fluor escein in the absence of live cells and the second is the quenching of fluo rescence by assay solutions. We show that common media components such as t ryptone, peptone and yeast extract all promote hydrolysis of FDA in the abs ence of live cells, as do Tris-HCl and sodium phosphate buffers. As a conse quence, Various microbiological media promote hydrolysis of FDA in the abse nce of live cells. Different media were also shown to reduce the amount of visible fluorescence of fluorescein. Diluting the medium decreases the back ground hydrolysis of FDA as well as increases the amount of visible fluores cence. Both problems should be considered when using FDA as an indicator of cell viability when testing natural products for antimicrobial activity. ( C) 2001 Elsevier Science B.V. All rights reserved.