Unique characteristics that help create specific styles of beer come from yeast strains and bacteria fermenting it.
Main beer fermenters are yeasts: ale yeast Saccharomyces cerevisiae ("top-fermenting" type since they rise to the surface during fermentation, creating a very thick, rich yeast head) and lager yeast (the "bottom-fermenting" type, Saccharomyces uvarum).
Yeast varieties such as Brettanomyces bruxellensis and Brettanomyces lambicus are common in lambics. In addition, other organisms such as Lactobacillus bacteria produce acids which contribute to the sourness. Pediococcus sours beers like lambics and Flanders.