culture of each isolate was grown in a nutrient broth medium containing 0.1 mg mL−1l-tryptophan and 5% NaCl and was incubated at 30◦C for 2–4 days. After incubation, the broth was centrifuged, the supernatant was retained and 1 mL of supernatant was mixed with 2 mL of Salkowski’s reagent (2%0.5 FeCl3 in 35% HCLO4 solution) and kept in the dark for minimum 30 min.