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→Introduction of Characteristics and Taxonomy
''Lactobacillus'' is a genus of bacteria that are considered to be a part of a broader classification of bacteria known as ''lactic acid bacteria'' (abbreviated as "LAB"). Other genera of bacteria that belong to this group and also appear in food fermentation include ''Lactococcus'', ''Streptococcus'', ''Pediococcus'', and ''Leuconostoc''. ''Lactobacillus'', as well as these other LAB genera, have three main metabolic pathways: glycolysis (fermentation of sugars), lopolysis (degradation of fat), and proteolysis (degradation of proteins). Lactic acid (specifically the conjugate base form, lactate), is the major byproduct of their fermentation. Other secondary metabolites include diacetyl, acetoin, acetaldehyde or acetic acid (some of which can contribute yogurt flavors to yogurt as well as maybe beer). While the lopolysis pathway contributes little to flavor, the proteolysis pathway produces amino acids which can be further converted into various alcohols, aldehydes, acids, esters, and sulphur compounds, many of which contribute various flavors to dairy fermentation products as well as to sour beer <ref name="Bintsis_2018">[http://www.aimspress.com/article/10.3934/microbiol.2018.4.665/fulltext.html Lactic acid bacteria as starter cultures: An update in their metabolism and genetics. Thomas Bintsis. 2018. DOI: 10.3934/microbiol.2018.4.665.]</ref>.
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" />. 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 <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>. 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>.
For more information on the metabolism of lactobacilli, see [[Lactobacillus#Metabolism|Metabolism]].