To confirm the presence of phase I in strain 1173, which exhibits a significantly higher level of antibiotic activity than phase II, the stock inoculum was spread on NBTA medium (45 g nutrient agar; 0. 025 g bromothymol blue; 0. 04 g 2,3,5-triphenyl-tetrazolium; 1,000 mL H2O) and incubated at 28±1°C for 48 h.