A metallic ring and 0.5 mL of MEM culture medium with 10% fetal calf serum were added to each well before cell seeding. Vero cells were seeded onto the 24-well culture plates at an initial density of 6 × 104 cells per well on both the BCG film and the control. Cells were incubated at 37 °C in a humidified atmosphere containing 5% CO2 for 16 h. Then, the culture medium was exchanged with serum-free MEM for another 48 h for the culture of Vero cells. At the end of incubation, the number of living cells was determined using the MTT assay.