yes, in curd making specially in farm condition, generally there always multiple strains are involved. Some strains are good at fermenting sugars to lactic acids. and some others are efficient producer of proteases to hydrolyze protein to amino acids & peptides. And some of these resulting peptides show antibacterial actions, are known as bacteriocin. Many types of bacteriocins are discovered and some are already in use for preserving foods, etc. So, it seems that types/activity/mode of hydrolysis etc. of proteases produced by a particular strain determine the type & potency of antibacterial agents it produces. Good luck