Dairy products like yogurt or cheese are manufactured using lactic acid bacteria.
Yogurt is made by the process that they sterilize properly concentrated non-fat milk by heat and add to it the lactic acid bacteria called a ‘starter’ to make them cultured in it. Generally Lactobacillus bulgaricus or Streptococcus lactis is used for fermentation, but the companies have been developing their own strain and using it for fermentation recently.
These lactic bacteria are not sterilized after the fermentation, so yogurt contains many lactic bacilli alive in it. And these bacteria have the function of suppressing the growth of harmful intestinal bacteria including coli. The lactic bacteria are also used as a starter in making cheese as in yogurt, but more various kinds of microorganisms are used in making cheese than in yogurt. There are the cheeses fermented by using mold, and of which surface color may look red or black.