NK activity assay
Quantitative analysis of NK activity was conducted by the fibrin plate method, with slight modification [17]. Briefly, bovine fibrinogen (Sigma, St. Louis, MO, USA) and thrombin (Sigma, St. Louis, MO, USA) were dissolved in 0.1 mol L-1 sodium phosphate buffer (PBS) at pH 7.4. An equi-voluminal mixture of 5 g L-1 bovine fibrinogen solution and 12 g L-1 agarose solution was then warmed in a 45°C bath, after which 10 μL of 500 U thrombin was added to 15 mL of the mixture solution in a 90 mm petri dish and kept at room temperature for 1 h to form fibrin. Holes with a 2 mm diameter were made on the fibrin plate and 10 µL of each diluted supernatant of fermentation broth was then added. The plates were subsequently incubated at 25°C for 16 h, after which the areas of the lysis zones on the fibrin plates were measured and the fibrinolytic activities were determined according to the standard curve of urokinase.