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As with some things in science that are not greatly explored, terminology isn't always agreed upon or fully established, and thus researching such a topic without a lot of prior knowledge can be challenging. Pellicles are a good example of this.
It is sometimes stated that a pellicle is a type of biofilm. However, there have been objections made to defining a pellicle as a type of biofilm. [http://www.biofilm.montana.edu/node/2390 Biofilms] are well studied and understood in science. Many microbiology textbooks define a biofilm as an aggregate of microorganisms where the cells adhere to each other on a solid surface <ref>[https://books.google.com/books?id=-ZDKAQAAQBAJ&pg=PT87&lpg=PT87&dq=brock+microbiology+biofilm&source=bl&ots=PwNZ20KFqq&sig=19lvlfqYj8bE7roiA7g4oJ3Xl0E&hl=en&sa=X&ved=0CCcQ6AEwAWoVChMImZaUmN_AxwIVVFmICh1pQgk0#v=onepage&q=brock%20microbiology%20biofilm&f=false e-Study Guide for Brock Biology of Microorganisms, textbook by Michael T. Madigan.]</ref>. Pellicles do not adhere to a solid surface, and are said to form at the "air-liquid interface" (surface of beer). As such they do not fit the definition of a biofilm in the strictest sense. However, the IUPAC, which is an international federation of National Adhering Organizations that works to standardize nomenclature in chemistry and other fields of science, defines a biofilm as an "aggregate of microorganisms in which cells that are frequently embedded within a self-produced matrix of extracellular polymeric substance (EPS) adhere to each other and/or to a surface <ref>[http://www.degruyter.com/view/j/pac.2012.84.issue-2/pac-rec-10-12-04/pac-rec-10-12-04.xml Pure and Applied Chemistry. Volume 84, Issue 2, Pages 377–410, ISSN (Online) 1365-3075, ISSN (Print) 0033-4545, DOI: 10.1351/PAC-REC-10-12-04, January 2012.]</ref>." According to the IUPAC, the "and/or" part of the definition allows for pellicles to be defined as biofilms since the cells adhere to each other. Although scientific definitions are not decided by a single or even multiple published studies, Lebleux et al. (2019) defines biofilm as, "Biofilms are complex associations of single- and multiple- species interconnected cells embedded in a hydrated self-produced matrix established at a solid/liquid or liquid/air interfaces." <ref>[https://www.sciencedirect.com/science/article/abs/pii/S0168160519303952 New advances on the Brettanomyces bruxellensis biofilm mode of life. Manon Lebleux, Hany Abdo, Christian Coelho, Louise Basmaciyan, Warren Albertin, Julie Maupeu, Julie Laurent, Chloé Roullier-Gall, Hervé Alexandre, Michèle Guilloux-Benatier, Stéphanie Weidmann, Sandrine Rousseaux. 2019. DOI: https://doi.org/10.1016/j.ijfoodmicro.2019.108464.]</ref>
Biofilms are extremely diverse and abundant in nature. Examples of biofilms in the classical sense include dental plaque and the green film produced by algae that covers stones in water streams <ref name="montana">[http://www.biofilm.montana.edu/node/2390 Center For Biofilm Engineering. Montona State University. Retrieved 09/05/2015.]</ref>. Biofilms are encountered in brewing in the form of contaminating microorganisms and poor cleaning/sanitation techniques. Biofilms are a common source of persistent brewhouse infections and can be resistant to the actions of many cleaning and sanitizing agents <ref name="montana"></ref><ref name="MTF_Thread"></ref>. Pellicles in beer do not attach to a solid surface, they appear on the "air-liquid interface" (the surface of the beer) <ref name="MTF_Thread">[https://www.facebook.com/groups/MilkTheFunk/permalink/1131334646894748/?comment_id=1131986723496207&offset=0&total_comments=57&comment_tracking=%7B%22tn%22%3A%22R3%22%7D Conversation about Pellicles on MTF. 08/20/2015.] </ref>. To make matters even more confusing, the two established definitions of a "pellicle" in biology only include the outer boundary of a protozoa cell <ref>[https://books.google.com/books?id=ACBcDjDLuNAC&pg=PA38&lpg=PA38&dq=pellicle+biology&source=bl&ots=58e43A_9h-&sig=ViakT4Ectu4DOlqoSWRCPFJ73iw&hl=en&sa=X&sqi=2&ved=0CGIQ6AEwDGoVChMIp6WkouTAxwIVgpmICh0F1wXO#v=onepage&q=pellicle%20biology&f=false Biology of Protozoa. D.R. Khanna. Discovery Publishing House, Jan 1, 2004. Pg 38.]</ref>, and the protein film that forms on the surface of teeth <ref>[https://en.wikipedia.org/wiki/Dental_pellicle Wikipedia. Dental Pellicle. Retrieved 08/23/2015.]</ref>.