is the name given to certain Gram-negative bacteria that occur as contaminants of pitching yeasts. The Irish brewing microbiologist J. L. Shimwell first isolated these bacteria in 1936 and, unsure how to classify them, assigned them to the genus Flavobacterium as F. proteus. In 1963 he realized that this was incorrect and created the monotypic genus Obesumbacterium. These bacteria were very common in beers until the 1980s and could be isolated from virtually every ale yeast in the UK, and less frequently from bottom-fermenting lager yeasts. During fermentation they become associated with the yeast head and are passed to the next fermentation when the yeast is re-pitched. They have relatively little impact on beer flavor when present in low numbers (less than 1% of the yeasts by number) but at higher populations can increase the dimethyl sulfide concentration to produce a fruity, parsnip-like odor. See dimethyl sulfide (dms).

During wort fermentation O. proteus reduces nitrate to nitrite, which can react with amines to form N-nitrosodimethylamine (nitrosamine). While the concentrations of nitrosamines so produced are minute and harmless, this led to brewers eliminating the contaminants using acid-washing procedures between fermentations. Together with improved yeast handling and better brewery hygiene this has made O. proteus very rare in breweries. In 2009 several strains from the 1970s were reexamined and shown to represent two very different genera of bacteria, both of which are members of the family Enterobacteriaceae. One retains the name Obesumbacterium proteus but the second has been placed in a new genus, Shimwellia (named after Shimwell) as Shimwellia pseudoproteus.