Relationship between PlyBt33 concentration and lytic activity. Lytic activities of PlyBt33 on viable cells of B. thuringiensis strain HD-73 with different PlyBt33 concentrations were tested. The initial OD600 of the strain suspension was 0.8 and the test was carried out at 37°C in 20 mM Tris-HCl (pH 8.0). The decrease of OD600 (%) = (1− the absorbance of the bacterial suspension at the end of each treatment / the absorbance at the beginning of each treatment) × 100%. The assay was carried out in triplicate and the mean values were used.