Primary factors affecting protoplast formation and regeneration of Lactobacillus bulgaricus and Streptococcus thermophilus were investigated using orthogonal array design to obtain fusants with bacteriophage resistance and superior fermentation performance. The results showed that the optimal preparation conditions for L. bulgaricus protoplasts were achieved by treatment with 1.0 mg/m L lysozyme for 30 min at 36 ℃ in the presence of hypertonic solution with phosphate buffer and mannitol. Under these conditi...