Different strains of bacteria are present in most breads and are one of the main agents responsible for the fermentation of naturally-leavened breads. They contribute a lot of flavour complexity through the production of different acids and help the fermentation of some substances that can't be broken down by yeasts.

The most common type of bacteria used in baking is lactic acid bacteria. That is, bacteria that produce lactic acid upon consumption of certain sugars. These bacteria are introduced to the dough through a starter, which is a colony of bacteria and yeasts maintained by bakers for the production of sourdough bread.