To ensure the presence of phase I, this strain was subcultured onto plates with NBTA media [nutrient agar (NA, g/L) consisting of peptone 10, beef extract 3, NaCl 5 and agar 15 supplemented with 0.04% triphenyltetrazolium chloride (w/v) and 0.025% bromothymol blue (w/v)] and incubated at 28°C in darkness.