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    • br Diseases and disorders caused by

    2018-11-06


    Diseases and disorders caused by alterations in the human gut microbiota It is evident that prenatal maternal exposure influences postnatal microbial colonization [3] and this plays pivotal roles in gut-associated lymphoid tissue (GALT) development [7], specific aspects of immune system development [8,9] and the integrity of the mucosal barrier [10]. Therefore the development of the gut microbiota in the early stages of life may be linked to future disease susceptibility. Many studies have associated diseases such as Inflammatory bowel disease [3], obesity [6] colon cancer [11] and some allergies [9] to alterations in the gut microbiome (Table 1). In many instances, there is an imbalance in the population densities of gut microbiota (dysbiosis) and this results in an overgrowth of pathogenic microbes. In obesity, the altered microbial population is associated with a shift in function of the cells, resulting in increased energy harvest from ingested food; unexpended excess energy is deposited as adipose tissue [12].
    Probiotics and probiotic selection criteria Probiotics are live microorganisms which, when administered in adequate amounts, confer health effects on the host and prebiotics are non-digestible food ingredients that stimulate the growth and or activity of probiotics [125]. Though some non-living cells may have probiotic properties [46,47] living cells tend to function better. The stomach is highly acidic due to the presence of HCl. Therefore, one of the first barriers probiotics must endure is the gastric acidity of the stomach as well as the bile in the upper digestive tract before they get to the small intestines [35]. Many types of bacteria have probiotic properties, however, the most documented groups comprise of lactic purchase Myoseverin bacteria (LAB) and bifidobacteria. While L. casei and Lactobacillus acidophilus survive in the acidic conditions of artificial gastric juice at pH 3.0 at 37°C, Lactobacillus delbruekii ssp. bulgaricus does not. Strains of Bifidobacterium vary in their ability to survive transit through the stomach. The initial screening and selection of probiotics also include testing of the phenotype and genotype stability, including plasmid stability; intestinal epithelial adhesion properties; protein and carbohydrate utilization patterns; production of antimicrobial substances; antibiotic resistance patterns; ability to inhibit known pathogens, spoilage organisms, or both; and immunogenicity [124]. Table 2 shows the properties and benefits of good probiotic strains. It is necessary that probiotic strains survive, proliferate and colonize their specific locations. They must neither be pathogenic nor trigger allergic response in the host. However, they may serve as adjuvants to stimulate the immune system against pathogens. Practically and for commercialization purposes, probiotics must be easily culturable on a large scale and must resist technological manipulations such as heating and low oxygen conditions in packages.
    Functional genomics of LAB Recently, the number of sequenced LAB genomes has increased exponentially and the genomic data from several LAB species and strains are available to give a better understanding of their gene content, their properties and their roles in human health and food fermentation [48]. The most important LAB used as starters in dairy fermentations are Lactococcus lactis, Streptococcus thermophilus, L. delbruekii subsp. bulgaricus, while in some cases also some Leuconostoc or other Lactobacillus spp. are used [49]. Because LAB do not contain a functional respiratory system, they obtain energy by substrate level phosphorylation. They use either the homofermentative pathway to virtually produce only lactate or the heterofermentative pathway to produce large amounts of CO2, and ethanol in addition to lactate. LAB compete with other bacteria based on their rapid growth and their lactic acid production in their habitats. Luesink et al. [50] have reported that the main factor controlling sugar degradation in LAB is the catabolite control protein CcpA which acts as a transcriptional activator of the lactic acid synthesis (las) operon with the order pfk-pyk-ldh. In many LAB, the ccpA gene is colocated with the prolidase-encoding pepQ gene but divergently transcribed from each other, indicating a link between carbon and nitrogen metabolism [2]. Of all the nitrogen control systems present in LAB, GlnR and CodY are the most studied. All LAB genomes posses GlnR but CodY is only present in Lactocccus, Streptococcus and Enterococcus spp. GlnR is involved in controlling the import of nitrogenous compounds and the synthesis of intracellular ammonia under high nitrogen concentrations [51], while CodY controls the proteolytic system of L. lactis and particularly the cell-wall proteinase (PrtP), the key enzyme in milk degradation [52].