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A '''pellicle''' is a type of biofilm which appears on the surface of beer (the classification of pellicles as a type of biofilm is up for debate. See [[Pellicle#Scientific_Terminology|Scientific Terminology]]). It consists of an aggregation of cells, proteins, and polymers <ref name="Branda"></ref><ref name="matt"></ref>. The "bubble" formations are caused by trapped CO2 beneath the pellicle film. Pellicles are often formed by ''[[Brettanomyces]]'', ''Acetobacter'' <ref name="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>, and other gram-negative bacteria such as some species of ''Acinetobacter'', ''Escherichia'', ''Burkholderia'', ''Dickeya'', ''Gluconacetobacter'', ''Pseudomonas'', ''Salmonella'', ''Shewanella'', and ''Vibrio''. Gram-positive bacteria ''Bacillus subtillis'' can also form a pellicle <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 (probably only wild species), and it might be possible for ''[[Lactobacillus]]'' and ''[[Pediococcus]]'' to form a pellicle (according to Matt Humbard; see reference) , however this has not been confirmed as far as we know <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>.