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==Overview==
Any microorganism that is introduced into a beer unintentionally and can survive in the beer is considered a "beer spoiler". 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 adapt to brewing conditions (namely hops) and to form biofilms and survive in beer and make a potential impact on the beer's flavor by producing acidity, phenols, turbidity/ropiness via exopolysaccharides (EPS), and/or super-attenuation (which can cause gushing or in extreme cases exploding bottles/cans) with just a few surviving cells. These effects can sometimes manifest days or even weeks after packaging, and longer storage or non-refrigerated storage can increase the potential for beer spoilers to negatively impact the beer. Bacteria species that have adapted to the brewing environment tend to be hop tolerant, but strains of the same species found outside of breweries are not tolerant of brewing conditions. It is thought that these species evolved to carry the genes to adapt to brewing conditions during the 5th to 9th centuries when hops were first being used in brewing, and that this evolution gave them a specialized adaption to the brewing environment where few competitors can survive <ref name="Suzuki_2012">[https://onlinelibrary.wiley.com/doi/abs/10.1002/j.2050-0416.2011.tb00454.x 125th Anniversary Review: Microbiological Instability of Beer Caused by Spoilage Bacteria. Ken Suzuki. 2012. DOI: https://doi.org/10.1002/j.2050-0416.2011.tb00454.x]</ref>.
Species of yeast and bacteria that are considered beer spoilers include [[Brettanomyces|''Brettanomyces'']] species, numerous [[Lactobacillus|''Lactobacillus'']] species, ''Pediococcus damnosus'', ''Pectinatus'' species (anaerobe responsible for 20-30% of bacterial contaminations that produces acetic acid, [https://en.wikipedia.org/wiki/Propionic_acid propionic acid], acetoin, and 'rotten egg' like odors in contaminated beer), ''Megasphaera cerevisiae'' (7% of bacterial contaminations; 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. Most wild yeasts that can grow in beer in lab conditions are not considered largely impactful because of their limitatons to growing in the presence of ethanol or lack of oxygen, but they can become impactful on barrel aged beers where oxygen is present (''Candida'' species, ''Pichia'' species, ''Torulaspora delbrueckii'', ''Issatchenkia orientalis'', ''Kluyveromyces marxianus'', ''Debaryomyces hansenii'', ''Zygosaccharomyces bailii'', ''Zygosaccharomyces bisporus'', ''Schizosaccharomyces pombe'', and ''Kloeckera apiculata'') <ref name="Bokulich_2013">[http://mmbr.asm.org/content/77/2/157.full The Microbiology of Malting and Brewing. Nicholas A. Bokulich and Charles W. Bamforth. 2013. DOI: 10.1128/MMBR.00060-12]</ref>. 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, and all lactic acid bacteria making up 60-90% of 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 cerevisiae'' strains (which sometimes produce phenols, haze, over-attenuation, and/or over-carbonation) 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 <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>. ''Pectinatus'' and ''Megasphaera'' are Gram-negative anaerobic species that produce a number of off-flavors in ales but not lagers (probably due to their preference for warmer temperatures). They are fairly hop tolerant (they can grow in beers with IBU's as high as 33-38) and often survive within the biofilms of other species in the brewing environment where the biofilm creates an anaerobic environment for them. They are sometimes found contaminating low ABV beers (under 5.2%) during packaging. They are not tolerant of pH below 4 and are killed at relatively low temperatures (58–60°C for one min) <ref name="Suzuki_2012" />. ''Zymomonas mobilis'' is a microaerophilic Gram-negative acetic acid bacteria that can withstand hops and can grow in bottled beer or casks where priming sugar is added and small amounts of air is present and produces high levels of acetaldehyde and hydrogen sulphide <ref name="Vaughan_2005" />.