title_body: What makes sour beers sour?
upvoted_answer: From the Wiki page on Sour Beer:
  
  "[S]our beers are made by intentionally allowing wild yeast strains or bacteria into the brew. Traditionally, Belgian brewers allowed wild yeast to enter the brew naturally through the barrels or during the cooling of the wort in a coolship open to the outside air – an unpredictable process that many modern brewers avoid.
  
  The most common agents used to intentionally sour beer are Lactobacillus, Brettanomyces, and Pediococcus. Another method for achieving a tart flavor is adding fruit during the aging process to spur a secondary fermentation or contribute microbes present on the fruit's skin."