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'''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>. 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]], 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.
==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 a 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, 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 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>.
For more information on the metabolism of ''Lactobacilli'', see [[Lactobacillus#Metabolism|Metabolism]].
==Commercial Lactobacillus Cultures==