Brewing yeasts are polyploid and belong to the Saccharomyces genera. The brewing strains can be classified into two groups; the ale strains (Saccharomyces cerevisiae) and the lager strains (Saccharomyces pastorianus or Saccharomyces carlsbergensis in the old taxonomy). Lager strains are a hybrid strain of S. cerevisiae and S. eubayanus and are often referred to as bottom fermenting. In contrast, ale strains are referred to as top fermenting strains, reflecting their separation characteristics in open square fermenters. Although the two species differ in a number of ways, including their response to temperature, sugar transport and use, the S. pastorianus and S. cerevisiae species are closely related within the Saccharomyces genus.
Saccharomyces yeasts can form symbiotic matrices with bacteria, and are used to produce kombucha, kefir and ginger beer.