If there's one trend in craft beer that has fought hardest to beat out the hoppy-hoppier-hoppiest IPA arms race, it's the boom in popularity of sour beers. Small production, time-intensive sour brews offer an intriguing history (and hype-inducing rarity), but it's their unique flavor that seems to turn most drinkers into dedicated sour beer fans. The tart, puckering taste is often met with a shocked, love-it-or-hate-it type of reaction, and those with the former can't seem to get enough of the stuff. The secret ingredients that set these beers apart from the rest of the brews on the shelf are actually living creatures: yeast and bacteria. Sour beers are typically made with a cocktail of little fermenting organisms, each one leaving a unique and identifiable fingerprint on your beer. Aside from Saccharomyces cerevisiae, the most common yeast used for brewing ales, sour beer brewers will often employ the wild yeast Brettanomyces, as well as bacteria such as Lactobacillus, Pediococcus, and Acetobacter.