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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 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'', ''P. frisingensis'', ''Megasphaera cerevisiae'', ''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. In sour beers with a pH below 4.3, only the 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>.
Biofilm forming spoilage organisms include a much wider range and higher frequency in beer tap systems than in brewhouses. This is due to the availability of oxygen and higher temperatures at certain points in the tap system, as well as poorer hygiene in tap systems as well as the difficulty to effectively clean plastic hoses. Of particular concern here is the ability of ''E. coli'' serotype O157:H7 to survive in tap systems, which has had a couple of documented occurrences in contaminated apple cider. Another study showed that aerobic yeasts were able to grow in dispensing lines, as well as ''L. brevis'', and in many cases the draft lines were re-contaminated one week after cleaning, indicating that a contamination in draft lines is difficult to remove <ref name="storgards_2000">[http://www.vtt.fi/inf/pdf/publications/2000/P410.pdf Process hygiene control in beer production and dispensing. Erna Storgårds. VTT Publications 410. 2000.]</ref><ref name="Wirtanen_2001" />.