Lambic beers, a speciality of Belgium, use wild, often airborne yeasts rather than cultivated ones. They are thought to be the oldest style of beer because before people understood the role of yeast in fermentation, they let it occur naturally. Many of the yeasts are not strains of brewer's yeast (Saccharomyces cerevisiae), and may produce significant differences in aroma and sourness in the beer. The natural yeast in the air varies from region to region, and therefore so do the lambics created in different locations. Yeast varieties such as Brettanomyces bruxellensis and Brettanomyces lambicus are quite common in lambics. In addition, other organisms such as Lactobacillus bacteria produce acids which contribute to the sourness.