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[[File:Omega-lacto-microscope.jpg|thumb|Omega Yeast Labs OYL-605 Lactobacillus Blend; photo by [https://www.facebook.com/groups/MilkTheFunk/permalink/1096077917087088/ Stephen Little].]]

'''Lactobacillus''' (often referred to by brewers as "Lacto") is a genus of Gram-positive, rod-shaped lactic acid bacteria (LAB) which produces acidity and sour flavors in the form of lactic acid and [[Lactobacillus#Sugar_Utilization_and_Secondary_Metabolites|secondary metabolites]] found in lambics, Berliner Weiss, sour brown ales, and gueuze. All ''Lactobacillus'' species are facultative anaerobes, which means they grow anaerobically but can also grow in the presence of oxygen and use oxygen to some degree <ref name="todar_lactics4"></ref>. They [https://www.researchgate.net/post/How_to_prepare_spore_forming_media_for_lactobacillus do not form spores]. There are more than 100 species, many of which are found in the human gastrointestinal track <ref name="todar_lactics4">[http://textbookofbacteriology.net/lactics_4.html ''Lactic Acid Bacteria''. Todar's Online Texbook of Bacteriology. Kenneth Todar, PhD. Pg. 4. Retrieved 07/28/2015.]</ref><ref name="Todar_nutgro4">[http://textbookofbacteriology.net/nutgro_4.html ''Nutrition and Growth of Bacteria''. Todar's Online Texbook of Bacteriology. Kenneth Todar, PhD. Retrieved 07/28/2015.]</ref>. In addition to beer, some species of ''Lactobacillus'' are also used to ferment yogurt, cheese, sauerkraut, pickles, wine, cider, kimchi, cocoa, and kefir <ref>[https://en.wikipedia.org/wiki/Lactobacillus ''Lactobacillus''. Wikipedia. Retrieved 07/28/2015.]</ref>. ''Lactobacillus'' can form a [[pellicle]] (need reference). See ''[[Pediococcus]]'', ''[[Brettanomyces]]'', ''[[Saccharomyces]]'', [[Mixed Cultures]], [[Kveik#Commercial_Availability|Kveik]], and [[Nonconventional Yeasts and Bacteria]] charts for other commercially available cultures. See the [[Wort Souring]] and [[Mixed Fermentation]] pages for brewing techniques with ''Lactobacillus''. See the [[Alternative Bacteria Sources]] section for culturing ''Lactobacillus'' from grains, yogurt, probiotics, and other sources.

The genus ''Lactobacillus'' contains a large number of relatively diverse species, and is the largest genus of the lactic acid bacteria group with over 50 species <ref>[https://web.archive.org/web/20070202132806/http://www.bacterio.cict.fr/l/lactobacillus.html List of Prokaryotic Names with Standing in Nomenclature - Genus Lactobacillus. J.P. Euzéby. Archive.org Wayback Machine; Feb 02, 2007.]</ref>, many of which have been identified as playing an important role in food fermentation or as probiotic species found in the human gut. The species ''Lactobacillus delbruekii'' consists of three subspecies: subsp. ''delbrueckii'', subsp. ''lactis'' and subsp. ''bulgaricus'', and have been used in yogurt fermentation. ''L. plantarum'' has one of the largest genomes among LAB. ''L. sanfranciscensis'' is the predominant LAB in sourdough cultures. ''Lactobacillus paracasei'' subsp. ''paracasei'', ''L. plantarum'', ''L. curvatus'', ''L. rahmosus'', and ''L. casei'' are often found in cheese maturation. ''L. johnsonii'' and ''L. reuteri'' strains have mostly been found in human and animal feces, suggesting that they are natural intestinal flora and are probiotic. Other species that have been used as probiotics include ''L. fermentum'', ''L. plantarum'', ''L acidophilis'' (the latter is also used in yogurt fermentation). ''Lactobacillus sakei'' subsp. ''sakei'' is used in the fermentation of sake <ref name="Bintsis_2018" />. Many of the previously mentioned species are purchased from yeast labs and used intentionally by brewers making sour beer (see [[Lactobacillus#Culture_Charts|Culture Charts]] below). ''L. acetotolerans'' has recently been claimed to also be found in many mixed fermentation sour beers, specifically in spontaneously fermented sour beers <ref>[https://www.sciencedirect.com/science/article/pii/S0740002020302471? Alexander Tyakht, Anna Kopeliovich, Natalia Klimenko, Daria Efimova, Nikita Dovidchenko, Vera Odintsova, Mikhail Kleimenov, Stepan Toshchakov, Alexandra Popova, Maria Khomyakova, Alexander Merkel. Characteristics of bacterial and yeast microbiomes in spontaneous and mixed-fermentation beer and cider, Food Microbiology. Volume 94, 2021, 103658.ISSN 0740-0020. https://doi.org/10.1016/j.fm.2020.103658.]</ref><ref>[https://www.biorxiv.org/content/10.1101/2021.07.21.453094v1 Mixed culture metagenomics of the microbes making sour beer. Renan Eugênio Araujo Piraine, Fábio Pereira Leivas Leite, Matthew L. Bochman. bioRxiv 2021.07.21.453094; doi: https://doi.org/10.1101/2021.07.21.453094.]</ref> (see also [https://www.facebook.com/groups/592560317438853/search/?q=acetotolerans MTF threads]).

Recently, whole genome sequencing led to the genetically driven proposal to divide the genus of ''Lactobacillus'' into either 2 subdivisions, or more radically into 10-14 subdivisions by one study <ref>[https://aem.asm.org/content/84/17/e00993-18 Comparative Genomics of the Genus Lactobacillus Reveals Robust Phylogroups That Provide the Basis for Reclassification. Elisa Salvetti, Hugh M. B. Harris, Giovanna E. Felis, Paul W. O'Toole. 2018. DOI: 10.1128/AEM.00993-18.]</ref><ref>[https://aem.asm.org/content/85/3/e02155-18 Towards a Genome-Based Reclassification of the Genus Lactobacillus. Stijn Wittouck, Sander Wuyts, Sarah Lebeer. 2019. DOI: 10.1128/AEM.02155-18.]</ref> and 23 divisions by another study accepted for publication by the International Journal of Systematic and Evolutionary Microbiology which tends to carry more authority in microbiological circles <ref>[https://www.microbiologyresearch.org/content/journal/ijsem/10.1099/ijsem.0.004107 A taxonomic note on the genus Lactobacillus: Description of 23 novel genera, emended description of the genus Lactobacillus Beijerinck 1901, and union of Lactobacillaceae and Leuconostocaceae. Jinshui Zheng et al. 2020. DOI: https://doi.org/10.1099/ijsem.0.004107.]</ref>. With the emergence of whole genome sequencing, other changes have been proposed, such as merging and splitting species of ''Lactobacillus'' <ref>[https://www.biorxiv.org/content/biorxiv/early/2019/01/31/537084.full.pdf A genome-based species taxonomy of the Lactobacillus Genus Complex. Stijn Wittouck, Sander Wuyts, Conor J Meehan, Vera van Noort, Sarah Lebeer. 2019. DOI: http://dx.doi.org/10.1101/537084.]</ref>. Renaming 200+ lactobacilli into new categories and names could also have a significant impact on the industries that use these microbes. The scale of this change has been discussed and considerations given for such industries, while the new classifications should be robust enough to withstand future scientific discoveries and should be based on genetic patterns <ref>[https://www.sciencedirect.com/science/article/pii/S0924224419303164 The potential impact of the Lactobacillus name change: the results of an expert meeting organised by the Lactic Acid Bacteria Industrial Platform (LABIP). Bruno Pot, Elisa Salvetti, Paola Mattarelli, Giovanna E. Felis. 2019. DOI: https://doi.org/10.1016/j.tifs.2019.07.006.]</ref>. Several species mergers and splits have also been identified <ref>[https://search.proquest.com/docview/2299499440?pq-origsite=gscholar A Genome-Based Species Taxonomy of the Lactobacillus Genus Complex. Wittouck Stijn; Wuyts Sander; Meehan, Conor J; van Noort Vera; Lebeer, Sarah. 2019. DOI:10.1128/mSystems.00264-19.]</ref>.