Pediococcus is a genus of coccoid-shaped (spherical) lactic acid bacteria. Pediococci typically occur in pairs and tetrads and were originally confused with the genus Sarcina, hence the term “sarcina sickness” for beer contaminated with these bacteria. Like other lactic acid bacteria, these Gram-positive organisms have a fermentative physiology (producing lactic acid from sugars) and grow best at low pH (around pH 4–5) and in the absence of air. It is for these reasons that specialized culture media set at a low pH and supplemented with glucose or sucrose are required for their growth in the laboratory.

Pediococci are associated with various food fermentations including vegetable pickles, sausages, and milk products. Of the 16 recognized species, Pediococcus damnosus is by far the most commonly encountered beer contaminant, probably because it has evolved tolerance of hop iso-alpha acids using a variety of molecular mechanisms. See iso-alpha acids. These include several different resistance genes similar to those that confer antibiotic resistance on bacteria and code for proteins referred to as multidrug exporters. P damnosus may be detected in the late fermentation and conditioning or packaged beer but rarely in pitching yeast. Contaminated beer is characterized by the buttery aroma of diacetyl, as well as turbidity and acid formation. Pediococci may produce copious amounts of extracellular polysaccharide, which forms viscous precipitate or “rope” in beers when some fermentable sugar is present.

See also lactic acid.