Lactic Acid Bacteria
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Lactobacillales are an order of
gram-positive In bacteriology, gram-positive bacteria are bacteria that give a positive result in the Gram stain test, which is traditionally used to quickly classify bacteria into two broad categories according to their type of cell wall. The Gram stain is ...
, low-GC, acid-tolerant, generally nonsporulating, nonrespiring, either rod-shaped (
bacilli Bacilli is a Taxonomy (biology), taxonomic Class (biology), class of bacteria that includes two orders, Bacillales and Lactobacillales, which contain several well-known pathogens such as ''Bacillus anthracis'' (the cause of anthrax). ''Bacilli'' ...
) or spherical ( cocci)
bacteria Bacteria (; : bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of Prokaryote, prokaryotic microorganisms. Typically a few micr ...
that share common
metabolic Metabolism (, from ''metabolē'', "change") is the set of life-sustaining chemical reactions in organisms. The three main functions of metabolism are: the conversion of the energy in food to energy available to run cellular processes; the ...
and
physiological Physiology (; ) is the science, scientific study of function (biology), functions and mechanism (biology), mechanisms in a life, living system. As a branches of science, subdiscipline of biology, physiology focuses on how organisms, organ syst ...
characteristics. These bacteria, usually found in decomposing plants and milk products, produce
lactic acid Lactic acid is an organic acid. It has the molecular formula C3H6O3. It is white in the solid state and it is miscible with water. When in the dissolved state, it forms a colorless solution. Production includes both artificial synthesis as wel ...
as the major metabolic end product of
carbohydrate A carbohydrate () is a biomolecule composed of carbon (C), hydrogen (H), and oxygen (O) atoms. The typical hydrogen-to-oxygen atomic ratio is 2:1, analogous to that of water, and is represented by the empirical formula (where ''m'' and ''n'' ...
fermentation Fermentation is a type of anaerobic metabolism which harnesses the redox potential of the reactants to make adenosine triphosphate (ATP) and organic end products. Organic molecules, such as glucose or other sugars, are catabolized and reduce ...
, giving them the common name lactic acid bacteria (LAB). Production of lactic acid has linked LAB with food fermentations, as acidification inhibits the growth of spoilage agents. Proteinaceous
bacteriocins Bacteriocins are proteinaceous or peptidic toxins produced by bacteria to inhibit the growth of similar or closely related bacterial strain(s). They are similar to yeast and paramecium killing factors, and are structurally, functionally, and ec ...
are produced by several LAB strains and provide an additional hurdle for spoilage and
pathogen In biology, a pathogen (, "suffering", "passion" and , "producer of"), in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a Germ theory of d ...
ic microorganisms. Furthermore, lactic acid and other metabolic products contribute to the
organoleptic Organoleptic properties are the aspects of food, water or other substances as apprehended via the senses—including taste, sight, smell, and touch. In traditional U.S. Department of Agriculture meat and poultry inspections, inspectors p ...
and textural profile of a food item. The industrial importance of the LAB is further evidenced by their
generally recognized as safe Generally recognized as safe (GRAS) is a United States Food and Drug Administration (FDA) designation that a chemical or substance added to food is considered safe by experts under the conditions of its intended use. An ingredient with a GRAS d ...
(GRAS) status, due to their ubiquitous appearance in food and their contribution to the healthy microbiota of animal and human mucosal surfaces. The
genera Genus (; : genera ) is a taxonomic rank above species and below family as used in the biological classification of living and fossil organisms as well as viruses. In binomial nomenclature, the genus name forms the first part of the binomial s ...
that comprise the LAB are at its core ''
Lactobacillus ''Lactobacillus'' is a genus of gram-positive, aerotolerant anaerobes or microaerophilic, rod-shaped, non-spore-forming bacteria. Until 2020, the genus ''Lactobacillus'' comprised over 260 phylogenetically, ecologically, and metabolically div ...
'', '' Leuconostoc'', '' Pediococcus'', '' Lactococcus'', and ''
Streptococcus ''Streptococcus'' is a genus of gram-positive spherical bacteria that belongs to the family Streptococcaceae, within the order Lactobacillales (lactic acid bacteria), in the phylum Bacillota. Cell division in streptococci occurs along a sing ...
'', as well as the more peripheral '' Aerococcus'', '' Carnobacterium'', ''
Enterococcus ''Enterococcus'' is a large genus of lactic acid bacteria of the phylum Bacillota. Enterococci are Gram-positive cocci that often occur in pairs ( diplococci) or short chains, and are difficult to distinguish from streptococci on physical ch ...
'', '' Oenococcus'', '' Sporolactobacillus'', '' Tetragenococcus'', '' Vagococcus'', and '' Weissella''. All but '' Sporolactobacillus'' are members of the Lactobacillales order, and all are members of the
Bacillota The Bacillota (synonym Firmicutes) are a phylum of bacteria, most of which have Gram-positive cell wall structure. They have round cells, called cocci (singular coccus), or rod-like forms (bacillus). A few Bacillota, such as '' Megasphaera'', ...
phylum. Although lactic acid bacteria are generally associated with the order Lactobacillales, bacteria of the genus '' Bifidobacterium'' (phylum
Actinomycetota The Actinomycetota (or Actinobacteria) are a diverse phylum of Gram-positive bacteria with high GC content. They can be terrestrial or aquatic. They are of great importance to land flora because of their contributions to soil systems. In soil t ...
) also produce lactic acid as the major product of carbohydrate metabolism.


Characteristics

The lactic acid bacteria (LAB) are either rod-shaped (
bacilli Bacilli is a Taxonomy (biology), taxonomic Class (biology), class of bacteria that includes two orders, Bacillales and Lactobacillales, which contain several well-known pathogens such as ''Bacillus anthracis'' (the cause of anthrax). ''Bacilli'' ...
), or spherical ( cocci), and are characterized by an increased tolerance to acidity (low pH range). This aspect helps LAB to outcompete other bacteria in a natural
fermentation Fermentation is a type of anaerobic metabolism which harnesses the redox potential of the reactants to make adenosine triphosphate (ATP) and organic end products. Organic molecules, such as glucose or other sugars, are catabolized and reduce ...
, as they can withstand the increased acidity from organic acid production (e.g.,
lactic acid Lactic acid is an organic acid. It has the molecular formula C3H6O3. It is white in the solid state and it is miscible with water. When in the dissolved state, it forms a colorless solution. Production includes both artificial synthesis as wel ...
). Laboratory media used for LAB typically include a
carbohydrate A carbohydrate () is a biomolecule composed of carbon (C), hydrogen (H), and oxygen (O) atoms. The typical hydrogen-to-oxygen atomic ratio is 2:1, analogous to that of water, and is represented by the empirical formula (where ''m'' and ''n'' ...
source, as most species are incapable of respiration. LAB are
catalase Catalase is a common enzyme found in nearly all living organisms exposed to oxygen (such as bacteria, plants, and animals) which catalyzes the decomposition of hydrogen peroxide to water and oxygen. It is a very important enzyme in protecting ...
-negative. LAB are amongst the most important groups of microorganisms used in the food industry. Their relative simple metabolism has also prompted their use as microbial cell factories for the production of several commodities for the food and non-food sectors


Metabolism

LAB genera are classified in terms of two main pathways of
hexose In chemistry, a hexose is a monosaccharide (simple sugar) with six carbon atoms. The chemical formula for all hexoses is , and their molecular weight is 180.156 g/mol. Hexoses exist in two forms, open-chain or cyclic, that easily convert into ...
fermentation: # Under conditions of excess
glucose Glucose is a sugar with the Chemical formula#Molecular formula, molecular formula , which is often abbreviated as Glc. It is overall the most abundant monosaccharide, a subcategory of carbohydrates. It is mainly made by plants and most algae d ...
and limited oxygen, homolactic LAB catabolize one mole of glucose in the Embden-Meyerhof-Parnas pathway to yield two moles of
pyruvate Pyruvic acid (CH3COCOOH) is the simplest of the alpha-keto acids, with a carboxylic acid and a ketone functional group. Pyruvate, the conjugate base, CH3COCOO−, is an intermediate in several metabolic pathways throughout the cell. Pyruvic ...
. Intracellular
redox Redox ( , , reduction–oxidation or oxidation–reduction) is a type of chemical reaction in which the oxidation states of the reactants change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is t ...
balance is maintained through the oxidation of
NADH Nicotinamide adenine dinucleotide (NAD) is a coenzyme central to metabolism. Found in all living cells, NAD is called a dinucleotide because it consists of two nucleotides joined through their phosphate groups. One nucleotide contains an ade ...
, concomitant with pyruvate reduction to lactic acid. This process yields two moles of ATP per mole of glucose consumed. Representative homolactic LAB genera include ''Lactococcus'', ''Enterococcus'', ''Streptococcus'', ''Pediococcus'', and group I lactobacilli # Heterofermentative LAB use the
pentose phosphate pathway The pentose phosphate pathway (also called the phosphogluconate pathway and the hexose monophosphate shunt or HMP shunt) is a metabolic pathway parallel to glycolysis. It generates NADPH and pentoses (five-carbon sugars) as well as ribose 5-ph ...
, alternatively referred to as the pentose phosphoketolase pathway. One mole of glucose-6-phosphate is initially dehydrogenated to 6-phosphogluconate and subsequently decarboxylated to yield one mole of CO2. 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 Ethanol (also called ethyl alcohol, grain alcohol, drinking alcohol, or simply alcohol) is an organic compound with the chemical formula . It is an Alcohol (chemistry), alcohol, with its formula also written as , or EtOH, where Et is the ps ...
via
acetyl-CoA Acetyl-CoA (acetyl coenzyme A) is a molecule that participates in many biochemical reactions in protein, carbohydrate and lipid metabolism. Its main function is to deliver the acetyl group to the citric acid cycle (Krebs cycle) to be oxidation, o ...
and
acetaldehyde Acetaldehyde (IUPAC systematic name ethanal) is an organic compound, organic chemical compound with the chemical formula, formula , sometimes abbreviated as . It is a colorless liquid or gas, boiling near room temperature. It is one of the most ...
intermediates. In theory, end products (including ATP) are produced in equimolar quantities from the
catabolism Catabolism () is the set of metabolic pathways that breaks down molecules into smaller units that are either oxidized to release energy or used in other anabolic reactions. Catabolism breaks down large molecules (such as polysaccharides, lipid ...
of one mole of glucose. Obligate heterofermentative LAB include ''Leuconostoc'', ''Oenococcus'', ''Weissella'', and group III lactobacilli Some members of ''Lactobacillus'' appear also able to perform
aerobic respiration Cellular respiration is the process of oxidizing biological fuels using an inorganic electron acceptor, such as oxygen, to drive production of adenosine triphosphate (ATP), which stores chemical energy in a biologically accessible form. Cellu ...
, making them facultative anaerobes, unlike the other members of the order, which are all aerotolerant. Using oxygen helps these bacteria deal with stress.


''Streptococcus'' reclassification

In 1985, members of the diverse genus ''Streptococcus'' were reclassified into '' Lactococcus'', ''
Enterococcus ''Enterococcus'' is a large genus of lactic acid bacteria of the phylum Bacillota. Enterococci are Gram-positive cocci that often occur in pairs ( diplococci) or short chains, and are difficult to distinguish from streptococci on physical ch ...
'', '' Vagococcus'', and ''
Streptococcus ''Streptococcus'' is a genus of gram-positive spherical bacteria that belongs to the family Streptococcaceae, within the order Lactobacillales (lactic acid bacteria), in the phylum Bacillota. Cell division in streptococci occurs along a sing ...
'' based on biochemical characteristics, as well as molecular features. Formerly, streptococci were segregated primarily based on
serology Serology is the scientific study of Serum (blood), serum and other body fluids. In practice, the term usually refers to the medical diagnosis, diagnostic identification of Antibody, antibodies in the serum. Such antibodies are typically formed in r ...
, which has proven to correlate well with the current taxonomic definitions. Lactococci (formerly Lancefield group N streptococci) are used extensively as fermentation starters in
dairy A dairy is a place where milk is stored and where butter, cheese, and other dairy products are made, or a place where those products are sold. It may be a room, a building, or a larger establishment. In the United States, the word may also des ...
production, with humans estimated to consume 1018 (one billion billion) lactococci annually. Partly due to their industrial relevance, both '' L. lactis'' subspecies (''L. l. lactis'' and ''L. l. cremoris'') are widely used as generic LAB models for research. ''L. lactis'' ssp. ''cremoris'', used in the production of hard
cheese Cheese is a type of dairy product produced in a range of flavors, textures, and forms by coagulation of the milk protein casein. It comprises proteins and fat from milk (usually the milk of cows, buffalo, goats or sheep). During prod ...
s, is represented by the laboratory strains LM0230 and MG1363. In similar manner, ''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 A genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding genes, other functional regions of the genome such as ...
of IL1403, which coincided with a significant shift of resources to understanding LAB
genomics Genomics is an interdisciplinary field of molecular biology focusing on the structure, function, evolution, mapping, and editing of genomes. A genome is an organism's complete set of DNA, including all of its genes as well as its hierarchical, ...
and related applications.


Phylogeny

The currently accepted taxonomy is based on the
List of Prokaryotic names with Standing in Nomenclature List of Prokaryotic names with Standing in Nomenclature (LPSN) is an online database that maintains information on the naming and taxonomy of prokaryotes, following the taxonomy requirements and rulings of the International Code of Nomenclatu ...
(LPSN) and
National Center for Biotechnology Information The National Center for Biotechnology Information (NCBI) is part of the National Library of Medicine (NLM), a branch of the National Institutes of Health (NIH). It is approved and funded by the government of the United States. The NCBI is lo ...
(NCBI).


Uses


Fermentation

Lactic acid bacteria are used in the food industry for a variety of reasons such as the production of
cheese Cheese is a type of dairy product produced in a range of flavors, textures, and forms by coagulation of the milk protein casein. It comprises proteins and fat from milk (usually the milk of cows, buffalo, goats or sheep). During prod ...
and
yogurt Yogurt (; , from , ; also spelled yoghurt, yogourt or yoghourt) is a food produced by bacterial Fermentation (food), fermentation of milk. Fermentation of sugars in the milk by these bacteria produces lactic acid, which acts on milk protein to ...
products. Popular drinks such as
kombucha Kombucha (also tea mushroom, tea fungus, or Manchurian mushroom when referring to the Microbiological culture, culture; Latin name ''Medusomyces gisevii'') is a fermented beverage, fermented, effervescent, Sweetened beverage, sweetened black ...
are made using lactic acid bacteria, with kombucha having been known to have traces of ''Lactobacillus'' and ''Pediococcus'' once the drink is made. The beer and wine-making process utilizes certain lactic acid bacteria, mostly ''Lactobacillus''. Lactic acid bacteria is used to start the wine-making process by starting the malolactic fermentation. After the malolactic fermentation, yeast cells are used to start the alcoholic fermentation process in grapes. The malolactic fermentation mechanism is mainly transformation of L-malic acid (dicarboxylic acid) to an lactic acid (monocarboxylic acid). This change occurs due to the presence of malolactic and malic enzymes. All malic acid are degraded and this increase the pH levels which changes the taste of the wine. Not only do they start the process but they are responsible for the different aromas produced in wine by the nutrients presence and the quality of the grapes. Also, the presence of different strains can change the desirability of aromas' presence. The different availability of enzymes that contribute to the vast spectrum of aromas in wine are associated with glycosidases, ''β''-glucosidases, esterases, phenolic acid decarboxylases and citrate lyases. By using molecular biology, researchers can help pick out different desirable strains that help improve the quality of wine and help with the removable of the undesirable strains. The same can be said about brewing beer as well which uses yeast with some breweries using lactic acid bacteria to change the taste of their beer.


Probiotics

Probiotics Probiotics are live microorganisms that provide health benefits when consumed, generally by improving or restoring the microbiota in the Gut microbiota, gut. Probiotics are considered GRAS, generally safe to consume, but may cause bacteria– ...
are products aimed at delivering living, potentially beneficial, bacterial cells to the gut
ecosystem An ecosystem (or ecological system) is a system formed by Organism, organisms in interaction with their Biophysical environment, environment. The Biotic material, biotic and abiotic components are linked together through nutrient cycles and en ...
of
human Humans (''Homo sapiens'') or modern humans are the most common and widespread species of primate, and the last surviving species of the genus ''Homo''. They are Hominidae, great apes characterized by their Prehistory of nakedness and clothing ...
s and other animals, whereas prebiotics are indigestible
carbohydrate A carbohydrate () is a biomolecule composed of carbon (C), hydrogen (H), and oxygen (O) atoms. The typical hydrogen-to-oxygen atomic ratio is 2:1, analogous to that of water, and is represented by the empirical formula (where ''m'' and ''n'' ...
s delivered in food to the large bowel to provide fermentable substrates for selected bacteria. Most strains used as probiotics belong to the genus ''
Lactobacillus ''Lactobacillus'' is a genus of gram-positive, aerotolerant anaerobes or microaerophilic, rod-shaped, non-spore-forming bacteria. Until 2020, the genus ''Lactobacillus'' comprised over 260 phylogenetically, ecologically, and metabolically div ...
''. (Other probiotic strains used belong to the genus '' Bifidobacterium''). Probiotics have been evaluated in research studies in animals and humans with respect to antibiotic-associated diarrhea, travellers' diarrhea, pediatric diarrhea,
inflammatory bowel disease Inflammatory bowel disease (IBD) is a group of inflammatory conditions of the colon and small intestine, with Crohn's disease and ulcerative colitis (UC) being the principal types. Crohn's disease affects the small intestine and large intestine ...
,
irritable bowel syndrome Irritable bowel syndrome (IBS) is a functional gastrointestinal disorder characterized by a group of symptoms that commonly include abdominal pain, abdominal bloating, and changes in the consistency of bowel movements. These symptoms may ...
and
Alzheimer's disease Alzheimer's disease (AD) is a neurodegenerative disease and the cause of 60–70% of cases of dementia. The most common early symptom is difficulty in remembering recent events. As the disease advances, symptoms can include problems wit ...
. Future applications of probiotics have been conjectured to include delivery systems for
vaccine A vaccine is a biological Dosage form, preparation that provides active acquired immunity to a particular infectious disease, infectious or cancer, malignant disease. The safety and effectiveness of vaccines has been widely studied and verifi ...
s and immunoglobulins, and the treatment of different gastrointestinal diseases and vaginosis.


Foods

The quest to find food ingredients with valuable bioactive properties has encouraged interest in
exopolysaccharide Extracellular polymeric substances (EPS) are natural polymers of high molecular weight secreted by microorganisms into their environment. EPS establish the functional and structural integrity of biofilms, and are considered the fundamental compo ...
s from LAB.
Functional food A functional food is a food claimed to have an additional benefit beyond just nutrition (often one related to health promotion or disease prevention) by modifying the horticulture, cultivation of the native food or by food additive, adding ingr ...
products that offer health and sensory benefits beyond their nutritional composition are becoming progressively more important to the food industry. The sensory benefits of exopolysaccharides are well established, and there is evidence for the health properties that are attributable to exopolysaccharides from LAB. However, there is a wide variation in molecular structures of exopolysaccharides and the complexity of the mechanisms by which physical changes in foods and bioactive effects are elicited. Some LAB produce bacteriocins which limit pathogens by interfering with cell wall synthesis or causing pore formation in the cell membrane.
Nisin Nisin is a polycyclic antibacterial peptide produced by the bacterium ''Lactococcus lactis'' that is used as a food preservative. It has 34 amino acid residues, including the uncommon amino acids lanthionine (Lan), methyllanthionine (MeLan), dideh ...
, a
bacteriocin Bacteriocins are proteinaceous or peptide, peptidic toxins produced by bacteria to inhibit the growth of similar or closely related bacterial strain(s). They are similar to yeast and paramecium killing factors, and are structurally, functionally ...
produced by LAB, was first researched as a food preservative in 1951 and has since been widely commercially used in foods due to its antimicrobial activity against Gram positive bacteria. Nisin is utilized as a food additive in at least 50 countries. In addition to having antibacterial activity, LAB can inhibit fungal growth. Various LAB, largely from genus '' Lactococcus'' and ''
Lactobacillus ''Lactobacillus'' is a genus of gram-positive, aerotolerant anaerobes or microaerophilic, rod-shaped, non-spore-forming bacteria. Until 2020, the genus ''Lactobacillus'' comprised over 260 phylogenetically, ecologically, and metabolically div ...
,'' suppress mycotoxigenic mold growth due to the production of anti-fungal metabolites. Furthermore, LAB have the potential to reduce the abundance of mycotoxins in foods by binding to them. In a study for postharvest food product safety conducted with 119 LAB isolated from the
rhizosphere The rhizosphere is the narrow region of soil or Substrate (biology), substrate that is directly influenced by root secretions and associated soil microorganisms known as the root microbiome. Pore space in soil, Soil pores in the rhizosphere can ...
of
olive trees The olive, botanical name ''Olea europaea'' ("European olive"), is a species of subtropical evergreen tree in the family Oleaceae. Originating in Asia Minor, it is abundant throughout the Mediterranean Basin, with wild subspecies in Africa ...
and desert truffles, mostly within the genera of ''
Enterococcus ''Enterococcus'' is a large genus of lactic acid bacteria of the phylum Bacillota. Enterococci are Gram-positive cocci that often occur in pairs ( diplococci) or short chains, and are difficult to distinguish from streptococci on physical ch ...
'' and '' Weissella'', researchers found strong antibacterial activity against '' Stenotrophomonas maltophilia'', '' Pantoea agglomerans'', '' Pseudomonas savastanoi'', ''
Staphylococcus aureus ''Staphylococcus aureus'' is a Gram-positive spherically shaped bacterium, a member of the Bacillota, and is a usual member of the microbiota of the body, frequently found in the upper respiratory tract and on the skin. It is often posi ...
'' and ''
Listeria monocytogenes ''Listeria monocytogenes'' is the species of pathogenic bacteria that causes the infection listeriosis. It is a facultative anaerobic bacterium, capable of surviving in the presence or absence of oxygen. It can grow and reproduce inside the ho ...
'', and anti-fungal activity against ''
Botrytis cinerea ''Botrytis cinerea'' is a necrotrophic fungus that affects many plant species, although its most notable hosts may be wine grapes. In viticulture, it is commonly known as "botrytis bunch rot"; in horticulture, it is usually called "grey mould" ...
'', '' Penicillium expansum'', '' Verticillium dahliae'' and ''
Aspergillus niger ''Aspergillus niger'' is a mold classified within the ''Nigri'' section of the ''Aspergillus'' genus. The ''Aspergillus'' genus consists of common molds found throughout the environment within soil and water, on vegetation, in fecal matter, on de ...
''.


Fertilizer

Researchers have studied the impact of lactic acid bacteria on indoleacetic acid production,
phosphate Phosphates are the naturally occurring form of the element phosphorus. In chemistry, a phosphate is an anion, salt, functional group or ester derived from a phosphoric acid. It most commonly means orthophosphate, a derivative of orthop ...
solubilization, and
nitrogen fixation Nitrogen fixation is a chemical process by which molecular dinitrogen () is converted into ammonia (). It occurs both biologically and abiological nitrogen fixation, abiologically in chemical industry, chemical industries. Biological nitrogen ...
on citrus. While most of the bacterial isolates were able to produce IAA, phosphate-solubilization was limited to only one of the eight LAB isolates.


Management of bacteriophages in industry

A broad number of food products, commodity chemicals, and
biotechnology Biotechnology is a multidisciplinary field that involves the integration of natural sciences and Engineering Science, engineering sciences in order to achieve the application of organisms and parts thereof for products and services. Specialists ...
products are manufactured industrially by large-scale bacterial fermentation of various organic substrates. Because this involves cultivating enormous quantities of bacteria each day in large fermentation vats, a serious threat in these industries is the risk of contamination by
bacteriophage A bacteriophage (), also known informally as a phage (), is a virus that infects and replicates within bacteria. The term is derived . Bacteriophages are composed of proteins that Capsid, encapsulate a DNA or RNA genome, and may have structu ...
s, which can rapidly bring fermentations to a halt and cause economical setbacks. Areas of interest in managing this risk include the sources of phage contamination, measures to control their propagation and dissemination, and biotechnological defense strategies developed to restrain them. In the context of the food fermentation industry, the relationship between bacteriophages and their bacterial hosts is very important. The dairy fermentation industry has openly acknowledged the problem of
phage A bacteriophage (), also known informally as a phage (), is a virus that infects and replicates within bacteria. The term is derived . Bacteriophages are composed of proteins that encapsulate a DNA or RNA genome, and may have structures tha ...
contamination, and has worked for decades with academia and starter-culture manufacturers to develop defence strategies and systems to curtail phages' propagation and evolution.


Bacteriophage–host interaction

The first contact between an infecting phage and its bacterial host is the phage's attaching 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. RBPs are also referred to as host-specificity proteins, host determinants, and antireceptors. A variety of molecules have been suggested to act as host receptors for
bacteriophage A bacteriophage (), also known informally as a phage (), is a virus that infects and replicates within bacteria. The term is derived . Bacteriophages are composed of proteins that Capsid, encapsulate a DNA or RNA genome, and may have structu ...
s infecting LAB; among those are
polysaccharide Polysaccharides (), or polycarbohydrates, are the most abundant carbohydrates found in food. They are long-chain polymeric carbohydrates composed of monosaccharide units bound together by glycosidic linkages. This carbohydrate can react with wat ...
s and (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 ranges. These studies, however, also found additional phage proteins to be important for successful phage infection. Analysis of the crystal structure of several RBPs indicates that these proteins share a common tertiary folding, and support previous indications of the
saccharide A carbohydrate () is a biomolecule composed of carbon (C), hydrogen (H), and oxygen (O) atoms. The typical hydrogen-to-oxygen atomic ratio is 2:1, analogous to that of water, and is represented by the empirical formula (where ''m'' and ''n'' m ...
nature of the host receptor.
Gram-positive In bacteriology, gram-positive bacteria are bacteria that give a positive result in the Gram stain test, which is traditionally used to quickly classify bacteria into two broad categories according to their type of cell wall. The Gram stain is ...
LAB have a thick
peptidoglycan Peptidoglycan or murein is a unique large macromolecule, a polysaccharide, consisting of sugars and amino acids that forms a mesh-like layer (sacculus) that surrounds the bacterial cytoplasmic membrane. The sugar component consists of alternating ...
layer, which must be traversed to inject the phage
genome A genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding genes, other functional regions of the genome such as ...
into the bacterial
cytoplasm The cytoplasm describes all the material within a eukaryotic or prokaryotic cell, enclosed by the cell membrane, including the organelles and excluding the nucleus in eukaryotic cells. The material inside the nucleus of a eukaryotic cell a ...
. Peptidoglycan-degrading enzymes are expected to facilitate this penetration, and such enzymes have been found as structural elements of a number of LAB phages.


Lactic acid bacteria and dental plaque

LAB are able to synthesize levans from
sucrose Sucrose, a disaccharide, is a sugar composed of glucose and fructose subunits. It is produced naturally in plants and is the main constituent of white sugar. It has the molecular formula . For human consumption, sucrose is extracted and refined ...
, and
dextran Dextran is a complex branched glucan (polysaccharide derived from the condensation of glucose), originally derived from wine. IUPAC defines dextrans as "Branched poly-α-d-glucosides of microbial origin having glycosidic bonds predominantly C-1 ...
s from
glucose Glucose is a sugar with the Chemical formula#Molecular formula, molecular formula , which is often abbreviated as Glc. It is overall the most abundant monosaccharide, a subcategory of carbohydrates. It is mainly made by plants and most algae d ...
. Dextrans, like other
glucan A glucan is a polysaccharide derived from D-glucose, linked by glycosidic bonds. Glucans are noted in two forms: alpha glucans and beta glucans. Many beta-glucans are medically important. They represent a drug target for antifungal medications of ...
, enable bacteria to adhere to the surface of teeth, which in turn can cause
tooth decay Tooth decay, also known as caries,The word 'caries' is a mass noun, and is not a plural of 'carie'.'' is the breakdown of teeth due to acids produced by bacteria. The resulting cavities may be a number of different colors, from yellow to black ...
through the formation of
dental plaque Dental plaque is a biofilm of microorganisms (mostly bacteria, but also fungi) that grows on surfaces within the mouth. It is a sticky colorless deposit at first, but when it forms Calculus (dental), tartar, it is often brown or pale yellow. It is ...
and production of lactic acid. While the primary bacteria responsible for tooth decay is ''
Streptococcus mutans ''Streptococcus mutans'' is a Facultative anaerobic organism, facultatively anaerobic, gram-positive coccus (round bacteria, bacterium) commonly found in the human oral cavity and is a significant contributor to dental caries, tooth decay. The m ...
'', LAB do feature among the other most common oral bacteria that cause decay.


Lactic acid bacteria genera

* '' Abiotrophia'' * '' Aerococcus'' * '' Carnobacterium'' * ''
Enterococcus ''Enterococcus'' is a large genus of lactic acid bacteria of the phylum Bacillota. Enterococci are Gram-positive cocci that often occur in pairs ( diplococci) or short chains, and are difficult to distinguish from streptococci on physical ch ...
'' * ''
Lactobacillus ''Lactobacillus'' is a genus of gram-positive, aerotolerant anaerobes or microaerophilic, rod-shaped, non-spore-forming bacteria. Until 2020, the genus ''Lactobacillus'' comprised over 260 phylogenetically, ecologically, and metabolically div ...
'' * '' Lactococcus'' * '' Leuconostoc'' * '' Oenococcus'' * '' Pediococcus'' * ''
Streptococcus ''Streptococcus'' is a genus of gram-positive spherical bacteria that belongs to the family Streptococcaceae, within the order Lactobacillales (lactic acid bacteria), in the phylum Bacillota. Cell division in streptococci occurs along a sing ...
'' * '' Tetragenococcus'' * '' Vagococcus'' * '' Weissella''


See also

* Gaffkaemia *
Malolactic fermentation Malolactic conversion (also known as malolactic fermentation or MLF) is a process in winemaking in which Tart (flavor), tart-tasting malic acid, naturally present in grape must, is converted to softer-tasting lactic acid. Malolactic fermentation ...
*
Lactic acid fermentation Lactic acid fermentation is a metabolic process by which glucose or other hexose, six-carbon sugars (also, disaccharides of six-carbon sugars, e.g. sucrose or lactose) are converted into cellular energy and the metabolite lactic acid, lactate, w ...
* Lacto-2 RNA motif *
List of bacterial orders This article lists the orders of the Bacteria. The currently accepted taxonomy is based on the List of Prokaryotic names with Standing in Nomenclature (LPSN) and National Center for Biotechnology Information (NCBI) and the phylogeny is based on 16 ...
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List of bacteria genera This article lists the genera of the bacteria Bacteria (; : bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of Prokaryote, ...
* Lacto-3 RNA motif


References


Further reading

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External links

* {{authority control Bacilli * Gram-positive bacteria sv:Lactobacillaceae