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Lactobacillus is a genus of Gram-positive, facultative anaerobic or microaerophilic, rod-shaped, non-spore-forming bacteria. They are a major part of the lactic acid bacteria group (i.e. they convert sugars to lactic acid). In humans, they constitute a significant component of the microbiota at a number of body sites, such as the digestive system, urinary system, and genital system. In women of European ancestry, Lactobacillus species are normally a major part of the vaginal microbiota. Lactobacillus forms biofilms in the vaginal and gut microbiota, allowing them to persist during harsh environmental conditions and maintain ample populations. Lactobacillus exhibits a mutualistic relationship with the human body as it protects the host against potential invasions by pathogens, and in turn, the host provides a source of nutrients. Lactobacillus is the most common probiotic found in food such as yogurt, and it is diverse in its application to maintain human well-being as it can help treat diarrhea, vaginal infections and skin disorders such as eczema.
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Many lactobacilli operate using homofermentative metabolism (they produce only lactic acid from sugars), and some species use heterofermentative metabolism (they can produce either alcohol or lactic acid from sugars). They are aerotolerant despite the complete absence of a respiratory chain. This aerotolerance is manganese-dependent and has been explored (and explained) in Lactobacillus plantarum. Many species of this genus do not require iron for growth and have an extremely high hydrogen peroxide tolerance.
The genomes of Lactobacillus are highly variable, ranging in size from 1.2 to 4.9 Mb (megabases). Accordingly, the number of protein-coding genes ranges from 1,267 to about 4,758 genes (in L. sanfranciscensis and L. parakefiri, respectively). Even within a single species there can be substantial variation. For instance, strains of L. crispatus have genome sizes ranging from 1.83 to 2.7 Mb, or 1,839 to 2,688 open reading frames.
Lactobacillus contains a wealth of compound microsatellites in the coding region of the genome, which are imperfect and have variant motifs.
The genus Lactobacillus currently contains over 180 species and encompasses a wide variety of organisms. The genus is polyphyletic, with the genus Pediococcus dividing the L. casei group, and the species L. acidophilus, L. salivarius, and L. reuteri being representatives of three distinct subclades. The genus Paralactobacillus falls within the L. salivarius group. In recent years, other members of the genus Lactobacillus (formerly known as the Leuconostoc branch of Lactobacillus) have been reclassified into the genera Atopobium, Carnobacterium, Weissella, Oenococcus, and Leuconostoc. More recently, the Pediococcus species P. dextrinicus has been reclassified as a Lactobacillus species. According to metabolism, Lactobacillus species can be divided into three groups:
- Obligately homofermentative (group I) including:
- Facultatively heterofermentative (group II) including:
- Obligately heterofermentative (group III) including:
The female genital tract is one of the principal colonisation sites for human microbiota, and there is interest in the relationship between the composition of these bacteria and human health, with a domination by a single species being correlated with general welfare and good outcomes in pregnancy. In around 70% of women, a Lactobacillus species is dominant, although that has been found to vary between American women of European origin and those of African origin, the latter group tending to have more diverse vaginal microbiota. Similar differences have also been identified in comparisons between Belgian and Tanzanian women.
Interactions with other pathogensEdit
Lactobacillus species produce hydrogen peroxide which inhibits the growth and virulence of the fungal pathogen Candida albicans in vitro and in vivo. In vitro studies have also shown that Lactobacillus sp. reduce the pathogenicity of C. albicans through the production of organic acids and certain metabolites. Both the presence of metabolites, such as sodium butyrate, and the decrease in environmental pH caused by the organic acids reduce the growth of hypha in C. albicans, which reduces its pathogenicity. Lactobacillus sp. also reduce the pathogenicity of C. albicans by reducing C. albicans biofilm formation. Biofilm formation is reduced by both the competition from Lactobacillus sp., and the formation of defective biofilms which is linked to the reduced hypha growth mentioned earlier. On the other hand, following antibiotic therapy, certain Candida species can suppress the regrowth of Lactobacillus sp. at body sites where they cohabitate, such as in the gastrointestinal tract.
In addition to its effects on C. albicans, Lactobacillus sp. also interact with other pathogens. For example, Lactobacillus reuteri can inhibit the growth of many different bacterial species by using glycerol to produce the antimicrobial substance called reuterin. Another example is Lactobacillus salivarius, which interacts with many pathogens through the production of salivaricin B, a bacteriocin.
Lactobacillus species administered in combination with other probiotics benefits cases of irritable bowel syndrome (IBS), although the extent of efficacy is still uncertain. The probiotics help treat IBS by returning homeostasis when the gut microbiota experiences unusually high levels of opportunistic bacteria. In addition, Lactobacillus species can be administered as probiotics during cases of infection by the ulcer-causing bacterium Helicobacter pylori. Helicobacter pylori is linked to cancer, and antibiotic resistance impedes the success of current antibiotic-based eradication treatments. When Lactobacillus probiotics are administered along with the treatment as an adjuvant, its efficacy is substantially increased and side effects may be lessened. Also, Lactobacillus is used to help control urogenital and vaginal infections, such as bacterial vaginosis (BV). Lactobacillus produce bacteriocins to suppress pathogenic growth of certain bacteria, as well as lactic acid and H2O2 (hydrogen peroxide). Lactic acid lowers the vaginal pH to around 4.5 or less, hampering the survival of other bacteria, and H2O2 reestablishes the normal bacterial flora and normal vaginal pH. In children, Lactobacillus strains such as L. rhamnosus are associated with a reduction of atopic eczema, also known as dermatitis, due to anti-inflammatory cytokines secreted by this probiotic bacteria.
Some Lactobacillus species have been associated with cases of dental caries (cavities). Lactic acid can corrode teeth, and the Lactobacillus count in saliva has been used as a "caries test" for many years. Lactobacilli characteristically cause existing carious lesions to progress, especially those in coronal caries. The issue is, however, complex, as recent studies show probiotics can allow beneficial lactobacilli to populate sites on teeth, preventing streptococcal pathogens from taking hold and inducing dental decay. The scientific research of lactobacilli in relation to oral health is a new field and only a few studies and results have been published. Some studies have provided evidence of certain Lactobacilli which can be a probiotic for oral health. Some species, but not all, show evidence in defense to dental caries. Due to these studies, there have been applications of incorporating such probiotics in chewing gum and lozenges. There is also evidence of certain Lactobacilli that are beneficial in the defense of periodontal disease such as gingivitis and periodontitis.
Some Lactobacillus species are used as starter cultures in industry for controlled fermentation in the production of yogurt, cheese, sauerkraut, pickles, beer, cider, kimchi, cocoa, kefir, and other fermented foods, as well as animal feeds. The antibacterial and antifungal activity of Lactobacillus species rely on production of bacteriocins and low molecular weight compounds that inhibits these microorganisms.
Sourdough bread is made either spontaneously, by taking advantage of the bacteria naturally present in flour, or by using a "starter culture", which is a symbiotic culture of yeast and lactic acid bacteria growing in a water and flour medium. The bacteria metabolize sugars into lactic acid, which lowers the pH of their environment, creating a signature "sourness" associated with yogurt, sauerkraut, etc.
In many traditional pickling processes, vegetables are submerged in brine, and salt-tolerant Lactobacillus species feed on natural sugars found in the vegetables. The resulting mix of salt and lactic acid is a hostile environment for other microbes, such as fungi, and the vegetables are thus preserved—remaining edible for long periods.
Lactobacilli, especially L. casei and L. brevis, are some of the most common beer spoilage organisms. They are, however, essential to the production of sour beers such as Belgian lambics and American wild ales, giving the beer a distinct tart flavor.
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Lactobacillus spp. convert tryptophan to indole-3-aldehyde (I3A) through unidentified enzymes . Clostridium sporogenes convert tryptophan to IPA , likely via a tryptophan deaminase. ... IPA also potently scavenges hydroxyl radicals
Table 2: Microbial metabolites: their synthesis, mechanisms of action, and effects on health and disease
Figure 1: Molecular mechanisms of action of indole and its metabolites on host physiology and disease
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IPA metabolism diagram
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Indole-3-propionate (IPA), a deamination product of tryptophan formed by symbiotic bacteria in the gastrointestinal tract of mammals and birds. 3-Indolepropionic acid has been shown to prevent oxidative stress and death of primary neurons and neuroblastoma cells exposed to the amyloid beta-protein in the form of amyloid fibrils, one of the most prominent neuropathologic features of Alzheimer's disease. 3-Indolepropionic acid also shows a strong level of neuroprotection in two other paradigms of oxidative stress. (PMID 10419516) ... More recently it has been found that higher indole-3-propionic acid levels in serum/plasma are associated with reduced likelihood of type 2 diabetes and with higher levels of consumption of fiber-rich foods (PMID 28397877)
Origin: • Endogenous • Microbial
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[Indole-3-propionic acid (IPA)] has previously been identified in the plasma and cerebrospinal fluid of humans, but its functions are not known. ... In kinetic competition experiments using free radical-trapping agents, the capacity of IPA to scavenge hydroxyl radicals exceeded that of melatonin, an indoleamine considered to be the most potent naturally occurring scavenger of free radicals. In contrast with other antioxidants, IPA was not converted to reactive intermediates with pro-oxidant activity.
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In addition, GI fungal infection is reported even among those patients with normal immune status. Digestive system-related fungal infections may be induced by both commensal opportunistic fungi and exogenous pathogenic fungi. ...
In vitro, bacterial hydrogen peroxide or organic acids can inhibit C. albicans growth and virulence61
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In vivo, C. albicans can suppress Lactobacillus sp. regeneration in the GI tract after antibiotic therapy63, 64
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