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==Overview==
While most microorganisms cannot survive in beer due to the hops, low pH, alcohol content, relatively high carbon dioxide, and shortage of nutrients, certain species are considered to be beer spoilage organisms due to their ability to form biofilms and survive in beer and make a potential impact on the beer's flavor by producing acidity, phenols, turbidity, and/or super-attenuation (which can cause gushing or in extreme cases exploding bottles/cans) with just a few surviving cells. Adaption to the brewing environment also makes them more able to survive the harsh environment of beer. These species include [[Brettanomyces|''Brettanomyces'']] species, numerous [[Lactobacillus|''Lactobacillus'']] species, ''Pediococcus damnosus'', ''Pectinatus cerevisiphilus'' (anaerobe that produces acetic acid, propionic acid, acetoin, and 'rotten egg' like odors in contaminated beer), ''P. frisingensis'', ''Megasphaera cerevisiae'' (inhibited below pH 4.1 and 2.8% ABV but can produce considerable amounts of [[Butyric Acid|butyric acid]] along with smaller amounts of acetic acid, caproic acid, [[Isovaleric Acid|isovaleric acid]], acetoin, and hydrogen sulphide), ''Selenomonas lactifex'', ''Zymophilus'' spp., [[Saccharomyces#Saccharomyces_cerevisiae_var._diastaticus|''Saccharomyces cerevisiae'' var. ''diastaticus'']], and some species from the ''Candida'' and ''Pichia'' genera. Hop tolerant lactic acid bacteria make up the majority of contamination issues in breweries, with ''L. brevis'' making up more than half of the reported contaminations. A new species of ''Lactobacillus'' was recently identified called ''L. acetotolerans'' and was [https://www.facebook.com/groups/MilkTheFunk/permalink/1363048380390039/ responsible for contaminating Goose Island's Bourbon County Stout], which is 60 IBU and 11% ABV. In sour beers with a pH below 4.3, only some lactic acid bacteria, ''Brettanomyces'', and some wild ''Saccharomyces'' have the potential for unwanted growth, while beers with low alcohol, a small amount of hops, lower CO<sup>2</sup> volumes (cask ales and beers dispensed with nitrogen, for example), and higher pH (4.4-4.6) are the most susceptible to contamination.
Other species of microbes do not grow in beer but can become contaminants earlier on in the brewing process (for example during kettle souring). These species include enterobacteria such as ''Clostridium'' species, ''Obesumbacterium proteus'' and ''Rahnella aquatilis'', and wild ''Saccharomyces'' that might not be able to grow in finished beer. Other species are considered "indicator" species because they do not directly cause spoilage of beer, but indicate that there is a hygiene problem. These include ''Acetobacter'', ''Gluconobacter'', and ''Klebsiella'' species, as well as aerobic yeasts, all of which usually don't have an impact when present unless oxygen is also present. They can also produce slime that protects other microorganisms that can have a greater impact on the beer's stability <ref name="Wirtanen_2001">[https://www.researchgate.net/publication/273439407_Disinfectant_testing_against_brewery-related_biofilms. Disinfectant testing against brewery-related biofilms. Erna Storgårds, Gun Wirtanen. 2001.]</ref><ref name="Bokulich_2018">[https://www.tandfonline.com/doi/abs/10.1094/ASBCJ-2012-0709-01 A Review of Molecular Methods for Microbial Community Profiling of Beer and Wine. Nicholas A. Bokulich, Charles W. Bamforth & David A. Mills. 2018.]</ref>. Some species can contaminate yeast pitches. ''Pediococcus damnosus'' is frequently the cause of such contaminations and can cause diacetyl problems, as well as ''Pediococcus inopinatus'', ''Selenomonas lacticifex'', and ''Zymophilus raffinosivorans'' (although these are rarely found in finished beer). ''Obesumbacterium proteus'' (which gives a parsnip-like smell and flavor) and ''Rahnella aquatilis'' can contaminate yeast pitches, and can inhibit fermentation and result in the beer finishing at a higher pH <ref name="Vaughan_2005">[https://onlinelibrary.wiley.com/doi/full/10.1002/j.2050-0416.2005.tb00221.x Enhancing the Microbiological Stability of Malt and Beer — A Review. Anne Vaughan, Tadhg O'Sullivan, Douwe Van Sinderen. 2005. DOI: https://doi.org/10.1002/j.2050-0416.2005.tb00221.x.]</ref>.