Lactic acid bacteria
Lactobacillales are an order of gram-positive, low-GC, acid-tolerant, generally nonsporulating, nonrespiring, either rod-shaped (bacilli) or spherical (cocci) bacteria that share common metabolic and physiological characteristics. These bacteria, usually found in decomposing plants and milk products, produce lactic acid as the major metabolic end product of carbohydrate fermentation, giving them the common name lactic acid bacteria (LAB).
Not to be confused with Lactobacillus.
Production of lactic acid has linked LAB with food fermentations, as acidification inhibits the growth of spoilage agents. Proteinaceous bacteriocins are produced by several LAB strains and provide an additional hurdle for spoilage and pathogenic 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 recognized as safe (GRAS) status, due to their ubiquitous appearance in food and their contribution to the healthy microbiota of animal and human mucosal surfaces.
The 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, Sporolactobacillus, Tetragenococcus, Vagococcus, and Weissella. All but Sporolactobacillus are members of the Lactobacillales order, and all are members of the Bacillota phylum.
Although lactic acid bacteria are generally associated with the order Lactobacillales, bacteria of the genus Bifidobacterium (phylum Actinomycetota) also produce lactic acid as the major product of carbohydrate metabolism.[1]
Characteristics[edit]
The lactic acid bacteria (LAB) are either rod-shaped (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, as they can withstand the increased acidity from organic acid production (e.g., lactic acid). Laboratory media used for LAB typically include a carbohydrate source, as most species are incapable of respiration. LAB are catalase-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]
Uses[edit]
Probiotics[edit]
Probiotics are products aimed at delivering living, potentially beneficial, bacterial cells to the gut ecosystem of humans and other animals, whereas prebiotics are indigestible carbohydrates delivered in food to the large bowel to provide fermentable substrates for selected bacteria. Most strains used as probiotics belong to the genus Lactobacillus. (Other probiotic strains used belong to the genus Bifidobacterium).[2][14]
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[15] and Alzheimer's disease.[16] Future applications of probiotics have been conjectured to include delivery systems for vaccines and immunoglobulins, and the treatment of different gastrointestinal diseases and vaginosis.[15]
Foods[edit]
The quest to find food ingredients with valuable bioactive properties has encouraged interest in exopolysaccharides 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.[17]
Some LAB produce bacteriocins which limit pathogens by interfering with cell wall synthesis or causing pore formation in the cell membrane.[18] Nisin, a bacteriocin 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.[19] Nisin is utilized as a food additive in at least 50 countries.[19] In addition to having antibacterial activity, LAB can inhibit fungal growth. Various LAB, largely from genus Lactococcus and Lactobacillus, suppress mycotoxigenic mold growth due to the production of anti-fungal metabolites.[20] Furthermore, LAB have the potential to reduce the abundance of mycotoxins in foods by binding to them.[20] In a study for postharvest food product safety conducted with 119 LAB isolated from the rhizosphere of olive trees and desert truffles, mostly within the genera of Enterococcus and Weissella, researchers found strong antibacterial activity against Stenotrophomonas maltophilia, Pantoea agglomerans, Pseudomonas savastanoi, Staphylococcus aureus and Listeria monocytogenes, and anti-fungal activity against Botrytis cinerea, Penicillium expansum, Verticillium dahliae and Aspergillus niger.[21]
Fertilizer[edit]
Researchers have studied the impact of lactic acid bacteria on indoleacetic acid production, phosphate solubilization, and nitrogen 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.[22]
Lactic acid bacteria and dental plaque[edit]
LAB are able to synthesize levans from sucrose, and dextrans from glucose.[28] Dextrans, like other glucan, enable bacteria to adhere to the surface of teeth, which in turn can cause tooth decay through the formation of dental plaque and production of lactic acid.[29] While the primary bacteria responsible for tooth decay is Streptococcus mutans, LAB do feature among the other most common oral bacteria that cause decay.[30]