Oberg CJ, Broadbent JR, Strickland M, McMahon DJ. Diversity in specificity of the extracellular proteinases in Lactobacillus helveticus and Lactobacillus delbrueckii subsp. bulgaricus. Lett Appl Microbiol. 2002;34(6):455–60. doi: 10.1046/j.1472-765x.2002.01123.x. PMID: 12028429.