Lactic acid bacteria 3907316 221984315 2008-06-27T00:15:10Z Sabedon 2083035 /* See also */ [[Category:Microbiology]] The '''Lactic Acid Bacteria (LAB)''' comprise a [[clade]] of [[Gram positive]], low-[[Guanine|G]][[Cytosine|C]], acid tolerant, non-sporulating, non-respiring rod or cocci that are associated by their common [[metabolic]] and [[physiological]] characteristics. These [[bacteria]], usually found in decomposing plants and lactic products produce [[lactic acid]] as the major metabolic endproduct of [[carbohydrate]] fermentation. This trait has historically linked LAB with [[Fermentation (food)|food fermentation]]s as acidification inhibits the growth of spoilage agents. Proteinaceous [[bacteriocins]] are produced by several LAB strains and provide an additional hurdle for spoilage and [[pathogen]]ic microorganisms. Furthermore, lactic acid and other metabolic products contribute to the organoleptic and textural profile of a food item. The industrial importance of the LAB is further evidenced by their generally regarded as safe ([[GRAS]]) status, due to their ubiquitous appearance in food and their contribution to the healthy microflora of human [[mucosal]] surfaces. The [[Genus|genera]] that comprise the LAB are at its core ''[[Lactobacillus]]'', ''[[Leuconostoc]]'', ''[[Pediococcus]]'', ''[[Lactococcus]]'', and ''[[Streptococcus]]'' as well as the more peripheral ''Aerococcus'', ''Carnobacterium'', ''[[Enterococcus]]'', ''[[Oenococcus]]'', ''Teragenococcus'', ''Vagococcus'', and ''Weisella''; these belong to the order [[Lactobacillales]]. {|align=left |__TOC__ |} ==Characteristics== The Lactic Acid Bacteria (LAB) are rod-shaped [[bacilli]] or [[coccus]]. LAB are characterized by an increased tolerance to a lower [[pH]] range. This aspect partially enables LAB to outcompete other bacteria in a natural [[Fermentation (biochemistry)|fermentation]], as they can withstand the increased acidity from organic acid production (e.g. [[lactic acid]]). Laboratory media used for LAB typically includes a [[carbohydrate]] source as most species are incapable of respiration. == LAB metabolism == There are two main [[hexose]] fermentation [[metabolic pathway|pathways]] that are used to classify LAB genera. Under conditions of excess [[glucose]] and limited oxygen, homolactic LAB catabolize one mole of glucose in the [[glycolysis|Embden-Meyerhof-Parnas]] (EMP) pathway to yield two [[mole (unit)|mole]]s of [[pyruvate]]. Intracellular [[redox]] balance is maintained through the oxidation of [[NADH]], concomitant with pyruvate reduction to lactic acid. This process yields two moles [[adenosine triphosphate|ATP]] per glucose consumed. Representative homolactic LAB genera include ''Lactococcus'', ''Enterococcus'', ''Streptococcus'', ''Pediococcus'' and group I lactobacilli. Heterofermentative LAB utilize the [[pentose phosphate pathway]], alternatively referred to as the pentose phosphoketolase pathway. One mole [[Glucose-6-phosphate]] is initially dehydrogenated to 6-phosphogluconate and subsequently decarboxylated to yield one mole of CO<sub>2</sub>. The resulting pentose-5-phosphate is cleaved into one mole glyceraldehyde phosphate (GAP) and one mole acetyl phosphate. GAP is further metabolized to lactate as in homofermentation, with the acetyl phosphate reduced to [[ethanol]] via [[acetyl-CoA]] and [[acetaldehyde]] intermediates. Theoretically, end products (including ATP) are produced in equimolar quantities from the [[catabolism]] of one mole glucose. Obligate heterofermentative LAB include ''Leuconostoc'', ''Oenococcus'', ''Weissella'', and group III lactobacilli. == ''Streptococcus'' reclassification == In [[1985]], members of the diverse genus ''Streptococcus'' were reclassified into ''Lactococcus'', ''Enterococcus'', ''Vagococcus'', and ''Streptococcus'' based on biochemical characteristics as well as molecular features. Historically, streptococci were segregated primarily based on [[serology]], which has proven to correlate well with the current taxonomic definitions. Lactococci (formerly Lancefield group N streptococci) are used extensively as [[starter (fermentation)|starter]] innocula in [[dairy]] fermentations, with humans estimated to consume 10<sup>18</sup> lactococci annually. Partly due to their industrial relevance, both ''Lactococcus lactis'' subspecies (''lactis'' and ''cremoris'') are widely used as generic LAB models for research. ''L. lactis'' ssp. ''cremoris'', used in the production of hard [[cheese]]s, is represented by the laboratory strains LM0230 and MG1363. Similarly, ''L. lactis'' ssp. ''lactis'' is employed in soft cheese fermentations, with the workhorse strain IL1403 ubiquitous in LAB research laboratories. In [[2001]], Bolotin et al sequenced the [[genome]] of IL1403 which coincided with a significant shift of resources to understanding LAB [[genomics]] and related applications. Currently, there are two ''L. lactis'' ssp. ''cremoris'' been sequenced that have been publicly released. ==Bacteriophages and LAB== A broad number of food products, commodity chemicals, and [[biotechnology]] products are manufactured industrially by large-scale bacterial [[fermentation]] of various organic substrates. Because enormous amounts of bacteria are being cultivated each day in large fermentation vats, the risk that [[bacteriophage]] contamination rapidly brings fermentations to a halt and cause economical setbacks is a serious threat in these industries. The relationship between bacteriophages and their bacterial hosts is very important in the context of the food fermentation industry. Sources of phage contamination, measures to control their propagation and dissemination, and biotechnological defence strategies developed to restrain phages are of interest. The dairy fermentation industry has openly acknowledged the problem of [[phage]] and has been working with academia and starter culture companies to develop defence strategies and systems to curtail the propagation and evolution of phages for decades.<ref name=McGrath>{{cite book | author = Mc Grath S and van Sinderen D (editors). | title = Bacteriophage: Genetics and Molecular Biology | edition = 1st ed. | publisher = Caister Academic Press | year = 2007 | url=http://www.horizonpress.com/phage | id = [http://www.horizonpress.com/phage ISBN 978-1-904455-14-1 ]}}</ref> ===[[Bacteriophage]] Host Interaction in LAB=== The first contact between an infecting [[phage]] and its bacterial host is the attachment of the [[phage]] to the host cell. This attachment is mediated by the phage's [[receptor binding protein]] (RBP), which recognizes and binds to a receptor on the bacterial surface. RBP's are also referred to as: host specificity protein, host determinant, and anti-receptor. For simplicity, the RBP term will be used here. A variety of molecules have been suggested to act as host receptors for [[bacteriophage]]s infecting LAB; among those are [[polysaccharide]]s, (lipo)teichoic acids as well as a single membrane protein. A number of RBPs of LAB phages have been identified by the generation of hybrid phages with altered host range. These studies, however, also found additional phage proteins to be important for successful a phage infection. Analysis of the crystal structure of several RBPs indicated that these proteins share a common tertiary folding as well as supporting previous indications of the [[saccharide]] nature of the host receptor. The [[Gram-positive]] LAB have a thick [[peptidoglycan]] layer, which must be traversed in order to inject the [[phage]] [[genome]] into the bacterial [[cytoplasm]]. Peptidoglycan-degrading enzymes are expected to facilitate this penetration and such enzymes have been found as structural elements of a number of LAB phages.<ref name=McGrath>{{cite book | author = Mc Grath S and van Sinderen D (editors). | title = Bacteriophage: Genetics and Molecular Biology | edition = 1st ed. | publisher = Caister Academic Press | year = 2007 | url=http://www.horizonpress.com/phage | id = [http://www.horizonpress.com/phage ISBN 978-1-904455-14-1 ]}}</ref> ==Probiotics and LAB== [[Probiotics]] are products aimed at delivering living, potentially beneficial, bacterial cells to the gut [[ecosystem]] of humans and other animals, whereas [[prebiotics]] are non-digestible [[carbohydrate]]s delivered in food to the large bowel to provide fermentable substrates for selected bacteria. Strains of LAB are the most common microbes employed as probiotics. Two principal kinds of probiotic/prebiotic bacteria, members of the genera ''[[Lactobacillus]]'' and ''[[Bifidobacterium]]'', have been studied in detail. <ref name=Tannock>{{cite book | author = Tannock G (editor). | title = Probiotics and Prebiotics: Scientific Aspects | edition = 1st ed. | publisher = Caister Academic Press | year = 2005 | url=http://www.horizonpress.com/pro3 | id = [http://www.horizonpress.com/pro3 ISBN 978-1-904455-01-8 ]}}</ref> ==References== {{reflist}} ==See also== * {{cite book | author = Holzapfel, WH; Wood, BJB (eds.). | title = The genera of lactic acid bacteria | edition = 1st ed. | year = 1998 | publisher = London Blackie Academic & Professional | id = ISBN 0-7514-0215-X}} * {{cite book | author = Salminen, S.; von Wright, A; and Ouwehand, AC (eds.). |title= Lactic Acid Bacteria: Microbiological and Functional Aspects | edition = 3rd ed. |year= 2004 |publisher= Marcel Dekker, Inc.|location= New York |id= ISBN 0-8247-5332-1}} * {{cite book | author = Madigan, Michael T.; Martinko, John M; and Parker, Jack|title= Brock. Biología de los Microorganismos| edition = 10th ed. |year= 2004 |publisher= Pearson Educaciòn S.A.|location= Madrid |id= ISBN 84-205-3679-2}} [[Category:Lactobacillales|*]] [[Category:Microbiology]] [[es:Cultivos lácticos]] [[ru:Молочнокислые бактерии]] [[fi:Maitohappobakteeri]] [[uk:Молочнокислі бактерії]]