| Lactic acid bacteria | |
|---|---|
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| Lesions ofWeissella confusa in themona monkey (hematoxylin andeosin stain): A)liver:portal triads withneutrophilic infiltration (x10); A1, presence of bacterialemboli inside thevein (arrow) (x40). B)acute pneumonia:edema,congestion, andleukocytecellsexudation in thepulmonary alveoli (x10). C)encephalitis: congestion andmarginalized neutrophils innervous vessels (x10) | |
Scientific classification | |
| Domain: | Bacteria |
| Kingdom: | Bacillati |
| Phylum: | Bacillota |
| Class: | Bacilli |
| Order: | Lactobacillales Ludwig, Schleifer & Whitman 2010 |
| Families | |
| Synonyms | |
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Lactobacillales are an order ofgram-positive,low-GC, acid-tolerant, generally nonsporulating,nonrespiring, either rod-shaped (bacilli) or spherical (cocci)bacteria that share commonmetabolic andphysiological characteristics. These bacteria, usually found in decomposing plants and milk products, producelactic acid as the major metabolic end product ofcarbohydratefermentation, giving them the common namelactic acid bacteria (LAB).
Production of lactic acid has linked LAB withfood fermentations, as acidification inhibits the growth of spoilage agents. Proteinaceousbacteriocins are produced by several LAB strains and provide an additional hurdle for spoilage andpathogenic microorganisms. Furthermore, lactic acid and other metabolic products contribute to theorganoleptic and textural profile of a food item. The industrial importance of the LAB is further evidenced by theirgenerally recognized as safe (GRAS) status, due to their ubiquitous appearance in food and their contribution to the healthy microbiota of animal and humanmucosal surfaces.
Thegenera that comprise the LAB are at its coreLactobacillus,Leuconostoc,Pediococcus,Lactococcus, andStreptococcus, as well as the more peripheralAerococcus,Carnobacterium,Enterococcus,Oenococcus,Sporolactobacillus,Tetragenococcus,Vagococcus, andWeissella. All butSporolactobacillus are members of the Lactobacillales order, and all are members of theBacillota phylum.
Although lactic acid bacteria are generally associated with the order Lactobacillales, bacteria of the genusBifidobacterium (phylumActinomycetota) also produce lactic acid as the major product of carbohydrate metabolism.[1]
The lactic acid bacteria (LAB) are either rod-shaped (bacilli), or spherical (cocci), and are characterized by an increased tolerance to acidity (lowpH range). This aspect helps LAB to outcompete other bacteria in a naturalfermentation, as they can withstand the increased acidity from organic acid production (e.g.,lactic acid). Laboratory media used for LAB typically include acarbohydrate source, as most species are incapable of respiration. LAB arecatalase-negative. LAB are amongst the most important groups of microorganisms used in the food industry.[2] 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[3]
LAB genera are classified in terms of two mainpathways ofhexose fermentation:
Some members ofLactobacillus appear also able to performaerobic respiration, making themfacultative anaerobes, unlike the other members of the order, which are all aerotolerant. Using oxygen helps these bacteria deal with stress.[5]
In 1985, members of the diverse genusStreptococcus were reclassified intoLactococcus,Enterococcus,Vagococcus, andStreptococcus based on biochemical characteristics, as well as molecular features. Formerly, streptococci were segregated primarily based onserology, which has proven to correlate well with the current taxonomic definitions. Lactococci (formerly Lancefield group N streptococci) are used extensively asfermentation starters indairy production, with humans estimated to consume 1018 (one billion billion) lactococci annually.[citation needed] Partly due to their industrial relevance, bothL. lactis subspecies (L. l. lactis andL. l. cremoris) are widely used as generic LAB models for research.L. lactis ssp.cremoris, used in the production of hardcheeses, 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, Bolotinet al. sequenced thegenome of IL1403, which coincided with a significant shift of resources to understanding LABgenomics and related applications.
The currently accepted taxonomy is based on theList of Prokaryotic names with Standing in Nomenclature (LPSN)[6] andNational Center for Biotechnology Information (NCBI).[7]
| 16S rRNA basedLTP_10_2024[8][9][10] | 120 marker proteins basedGTDB 09-RS220[11][12][13] |
|---|---|
Lactic acid bacteria are used in the food industry for a variety of reasons such as the production ofcheese andyogurt products. Popular drinks such askombucha are made using lactic acid bacteria, with kombucha having been known to have traces ofLactobacillus andPediococcus once the drink is made.[14]
The beer and wine-making process utilizes certain lactic acid bacteria, mostlyLactobacillus. 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 thealcoholic fermentation process in grapes. The malolactic fermentation mechanism is mainly transformation of L-malic acid (dicarboxylic acid) to an lactic acid (monocarboxylic acid).[15] 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.[15] 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.[16]
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.[17]
Probiotics are products aimed at delivering living, potentially beneficial, bacterial cells to the gutecosystem ofhumans and other animals, whereasprebiotics are indigestiblecarbohydrates delivered in food to the large bowel to provide fermentable substrates for selected bacteria. Most strains used as probiotics belong to the genusLactobacillus. (Other probiotic strains used belong to the genusBifidobacterium).[2][18]
Probiotics have been evaluated in research studies in animals and humans with respect to antibiotic-associated diarrhea, travellers' diarrhea, pediatric diarrhea,inflammatory bowel disease,irritable bowel syndrome[19] andAlzheimer's disease.[20] Future applications of probiotics have been conjectured to include delivery systems forvaccines and immunoglobulins, and the treatment of different gastrointestinal diseases andvaginosis.[19]
The quest to find food ingredients with valuablebioactive properties has encouraged interest inexopolysaccharides from LAB.Functional food 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.[21]
Some LAB produce bacteriocins which limit pathogens by interfering with cell wall synthesis or causing pore formation in the cell membrane.[22]Nisin, abacteriocin 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.[23] Nisin is utilized as a food additive in at least 50 countries.[23] In addition to having antibacterial activity, LAB can inhibit fungal growth. Various LAB, largely from genusLactococcus andLactobacillus, suppress mycotoxigenic mold growth due to the production of anti-fungal metabolites.[24] Furthermore, LAB have the potential to reduce the abundance of mycotoxins in foods by binding to them.[24] In a study for postharvest food product safety conducted with 119 LAB isolated from therhizosphere ofolive trees and desert truffles, mostly within the genera ofEnterococcus andWeissella, researchers found strong antibacterial activity againstStenotrophomonas maltophilia,Pantoea agglomerans,Pseudomonas savastanoi,Staphylococcus aureus andListeria monocytogenes, and anti-fungal activity againstBotrytis cinerea,Penicillium expansum,Verticillium dahliae andAspergillus niger.[25]
Researchers have studied the impact of lactic acid bacteria onindoleacetic acid production,phosphate solubilization, andnitrogen fixation 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.[26]
A broad number of food products, commodity chemicals, andbiotechnology 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 bybacteriophages, 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 ofphage contamination, and has worked for decades with academia and starter-culture manufacturers to develop defence strategies and systems to curtail phages' propagation and evolution.[27]
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 forbacteriophages infecting LAB; among those arepolysaccharides 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 thesaccharide nature of the host receptor.Gram-positive LAB have a thickpeptidoglycan layer, which must be traversed to inject the phagegenome into the bacterialcytoplasm. Peptidoglycan-degrading enzymes are expected to facilitate this penetration, and such enzymes have been found as structural elements of a number of LAB phages.[27]
LAB are able to synthesizelevans fromsucrose, anddextrans fromglucose.[28] Dextrans, like otherglucan, enable bacteria to adhere to the surface of teeth, which in turn can causetooth decay through the formation ofdental plaque and production of lactic acid.[29] While the primary bacteria responsible for tooth decay isStreptococcus mutans, LAB do feature among the other most commonoral bacteria that cause decay.[30]
