What is a "brewer's yeast" used in beer fermentation?

Brewer's yeast, which is essential for the fermentation process in beer production, is primarily represented by Saccharomyces cerevisiae. This strain of yeast is specifically cultivated for its ability to convert sugars from malted grains into alcohol and carbon dioxide through fermentation. Saccharomyces cerevisiae is known for its rapid fermentation qualities and the production of desirable flavors and aromas, making it the most commonly used yeast in brewing, especially for ale fermentation.

The other options refer to organisms that serve different functions in fermentation processes or are not traditionally associated with beer brewing. Escherichia coli is a type of bacteria often associated with food safety issues, not beer fermentation. Brettanomyces bruxellensis is another yeast variety, but it is typically used in creating specific wild or sour ales, contributing complex flavors; it is not the standard brewer's yeast. Lactobacillus bulgaricus is a lactic acid bacteria used mainly in yogurt production and certain sour beer styles, but it does not perform the primary fermentation needed for standard beer production. Thus, Saccharomyces cerevisiae remains the definitive choice for brewer's yeast in the context of traditional beer fermentation.