Each well of a 96 well ELISA plate was filled with 175 ìL of an exponentially growing culture (106-107 CFU/mL) and 20 ìL solution of each concentration of the test substance. The ELISA trays were incubated for 40 minutes before 5 ìL of FDA (2 mg/mL) was added. Incubation was continued for a further 90 minutes and the resulting green colour from the hydrolysis of FDA was measured at 490 nm (reference to 630 nm) and blank against control wells containing microbial cultures only.