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[[File:Brussels brett blend pellicle1.jpg|thumb|Pellicle from The Yeast Bay Brussels Brett Blend; courtesy of Dan Pixley]][[File:Lactobacillus pellicle.jpg|thumb|Lactobacillus pellicle; courtesy of Matt Humbard]] A '''pellicle''' is an aggregation of cells, proteins, and long chain sugars (polysaccharides) formed on the surface of a beer. The "bubble" formations are caused by trapped CO2 beneath the pellicle film. Pellicles are often formed by [[Brettanomyces]], [[Pediococcus]], and [[Lactobacillus]], as well as ''Acetobacter spp'' <refname="Perumpuli">[http://wwwir.ncbikagoshima-u.nlmac.nih.govjp/bitstream/10232/21479/pubmed4/24559734 Diss_Perumpuli+Arachchige_Buddhika+Niroshie_RNK999_2014.pdf.pdf Pellicle of thermotolerant Acetobacter pasteurianus strains: characterization Characterization of the polysaccharides polysaccharide and of the induction patterns. Perumpuli PA, Watanabe T, Toyama HArachchige Buddhika Niroshie. Aug 2014-09-30.]</ref> and other gram-negative bacteria <ref name="Armitano">[http://www.ncbi.nlm.nih.gov/pubmed/25756106 Gram-negative bacteria can also form pellicles. Armitano J, Méjean V, Jourlin-Castelli C. Environ Microbiol Rep. 2014 Dec.]</ref>. It can also be formed by [[Saccharomyces]] in rare occasions <ref name="matt">[http://phdinbeer.com/2015/01/30/beer-microbiology-what-is-a-pellicle/ Beer Microbiology – What is a pellicle? A PhD in Beer blog. Dr. Matt Humbard. 01/30/2015. Retrieved 04/26/2015.]</ref>.

''Acetobacter spp.'' produce homo and heteropolysaccharides (polysaccharides consisting of one type of sugar or more than one type of sugar, respectively <ref>[http://www.britannica.com/science/heteropolysaccharide Heteropolysaccharide. Encyclopedia Britannica. Retrieved 09/05/2015.]</ref>) that attach to the surface of the cells (capsular polysaccharides - '''CPS'''), as well as polysaccharides that are secreted into the medium in which they live (extracellular polysaccharides, or exopolysaccharides - '''EPS'''). The pellicle formation in ''Acetobacter'' is created by the collection of cells tightly associated to one another via the CPS on the cell walls<ref name="Perumpuli"></ref>..

The exact composition of the CPS polysaccharides within ''Acetobacter'' pellicles varies between not only species of ''Acetobacter'' and another acetic acid bacteria genus called ''Gluconacetobacter'', but also strains within species. For example, ''Gluconacetobacter xylinus'' produces a homopolysaccharide pellicle consisting of cellulose, ''A. pasteurianus'' subsp. Lovaniensis produces a heteropolysaccharide pellicle consisting of glucose and rhamnose, and ''A. tropicalis'' produces a heteropolysaccharide pellicle consisting of glucose, galactose, and rhamnos. The ratios of the different sugars in heteropolysaccharides was shown to vary from strain to strain of ''A. pasteurianus''. As pellicle formation increases, the structure of the polysaccharides that make it up do not change<ref name="Perumpuli"></ref>..

Pellicle formation in ''Acetobacter tropicalis'' has been linked to a gene cluster (polABCDE), and disruption of these gene switched the cells from producing CPS (and pellicle formation) to producing EPS instead<ref name="Perumpuli"></ref>..

The presence of ethanol in concentrations of 1-4% encourages pellicle production in some strains of ''Acetobacter pasteurianus'' (although the presence of ethanol encourages pellicle formation, the amount of ethanol did not make a difference). In some strains of ''A. pasteurianus'', sugar encourages pellicle formation. It has been suggested that CPS production in ''Acetobacter'' is a function of stress tolerance, since strains by acting as a barrier around the cell that protects it from acetic acid in the environment. Strains that form a pellicle in the presence of ethanol and/or higher temperatures can fully ferment vinegar whereas those that do not create a pellicle perform poorly in comparison <refname="Perumpuli">[http://ir.kagoshima-u.ac.jp/bitstream/10232/21479/4/Diss_Perumpuli+Arachchige_Buddhika+Niroshie_RNK999_2014.pdf.pdf Pellicle of thermotolerant Acetobacter pasteurianus strains: Characterization of polysaccharide and induction patterns. Perumpuli Arachchige Buddhika Niroshie. 2014-09-30.]</ref>.