Pasteurization does not sterilize the milk. Some pathogenic bacteria can survive pasteurization. Many nonpathogenic bacteria and bacterial spores will remain in the milk. Milk left standing will eventually sour because of these bacteria. Classically, this process is performed by bacteria.
Acid is essential to the production of good cheese.
However, if there is too much acid in the milk the cheese will be crumbly. If not enough acid is present the curd will be pasty. The most common method of acidifying milk is the use of bacteria. Bacteria feed on the lactose in milk and produce lactic acid as a waste product. With time, increasing amounts of lactic acid lower the pH of the milk.