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In addition to safety and technical aspects, functional screening plays a key role in the selection of potential probiotic strains for human use. In addition to their health-promoting role, functional aspects of probiotic strains include their ability to reach the colon as metabolically active cells. Novel probiotics should be selected based on their strain characteristics. In addition to the ability to inhibit the growth of pathogens, the main selection criteria for probiotics include many functional properties, such as tolerance to gastric acidity and bile toxicity, adhesion to intestinal epithelium cells and/or mucus, and ability to improve the immune response of the host. In vitro simulated gastric survival methods are most commonly used as the first step to evaluate new probiotic strains. These methods include incubations in acidified medium or simulated gastric juices, as well as more complex gastrointestinal models that simulate further considerations of GI transit. Preliminary characterization of probiotic culture strains by in vitro screening is of great value for the selection of functional candidates for further in vivo studies.
When microbial cells encounter a severely acidic environment, proton leakage becomes greater than the cells' ability to keep their normal homeostasis. Bacterial and yeast have evolved several mechanisms to withstand acidic environments including activation of H+-ATPases, cell envelope remodeling, intracellular proton consumption (glutamate decarboxylation), or alkali production (urease or arginine deiminase activities). To tolerate the presence of bile acids/salts, potential probiotics may synthesize proteins and fatty acids, produce protective biopolymers, use specific enzymes (BSH) to hydrolyze bile salts, and modify the composition of cell membranes. In addition, the mechanism of tolerance also includes the use of protein expression to metabolize carbohydrates and fatty acids, biosynthesis of nitrogenous bases and amino acids, and production of stress response proteins.
Fig.1 Schematic representation of common acid-resistance mechanisms in microorganisms. (Ayyash, 2021)
To confer health benefits to the host, probiotics need to reach the gut through harsh GI conditions such as low pH, pepsin, bile, and proteolytic enzymes. To survive and colonize the GI tract, bacteria should exhibit high tolerance to acidic media and bile and should be able to adhere to the intestinal surface. In acid tolerance tests, exposure time and pH are important characteristics affecting strain survival. Acid resistance is considered to be one of the desirable characteristics for screening potential probiotic strains.
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For Research Use Only. Not intended for use in food manufacturing or medical procedures (diagnostics or therapeutics). Do Not Use in Humans.