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Different pH tolerance of probiotic strain Lactobacillus plantarum GLB18 written by Rosita Tropcheva, PhD
The probiotic bacteria, being implemented in many different food products, get into the stomach and go through the different parts of gastrointestinal tract (GIT). They remain in the digestive system for a while, being exposed to very unfavorable conditions. The main criteria for selection of a probiotic strain is its ability to survive in the gastric transit and to provide (temporarily – for the transiting species and constantly for the colonizing species) benefits for the consumer. Therefore in the last years, the index for transit tolerance presenting the vitality of the strain in GIT plays a main role in the in vitro selection procedures of potential probiotic strains. According to this, the probiotic strain has been subjected to in vitro assay for assessment of the survival rate and activity into GIT.
Lactobacillus plantarum GLB18 was cultivated in MRS broth (Merck, USA) and in anaerobic conditions and at 37 degrees celsius.
Exponentially growing probiotic culture was inoculated in MRS broth with pH 2.0 (adjusted with 0.5 M HCl before sterilization), pH 6.0 (control sample) and pH 8.0 (adjusted with 10% NaOH before sterilization). All samples were incubated anaerobically at 37 degrees celsius. The tolerance of the tested strains was estimated by agar plate count tests at different time points during the 3 hours cultivation. Each assay was performed in triplicate.
Results: Growth and survival rate under conditions of different pH
The tolerance to a low pH is one of the most important criteria in the selection of probiotic species. The low pH and the presence of digestive enzymes in the stomach are the first barrier for the bacteria. The pH rates in the stomach of healthy human varies.
1.5-2.0 to рН 3.5 is unfavorable for the live bacteria and a lowering in the viability of the most lactobacilli has been reported. Food remains in the stomach from 30 minutes to 3 hours, depending on the individual metabolism and the diet. The assessment of the strain includes growth rate of the tested probiotic culture in different pH conditions, which simulates the changes in acid content in the GIT, by the time of digestion: рН 2.0 and рН 8.0.
In vitro estimation of the survival rate of probiotic strain in modified growth media with pH 2.0, pH 6.0 (control) and pH 8.0, cultivated for 3 hours at 37 degrees celsius.
The in vitro assay shows that Lactobacillus plantarum GLB18 is most sensitive to the acid content with pH rates over 2.0 (see Fig. 1). In spite of this fact, the bacteria have a stable population, which is determined. Growth media with pH 8.0 ensures bacteria growth comparable to the control sample. It could be concluded, that under stressful conditions the test probiotic strain GLB18 not only have good survival rate under low pH, but also increase their populations in the sample with high pH.
The retained viability for at least 3 hours is enough to classify a particular strain as a probiotic. In vitro time for transit through the stomach could be less than 1 hour depending on the type of food, the diet and the individual. Additionally, the probiotic strains included in different pharmaceutical products are not in suspension-like fоrm (as it is in this in vitro tests), but in symbiotic form containing a variety of additional substrates.
The probiotic bacteria are not directly exposed to the low pH in stomach and this is a precondition for even a better survival rate of the tested strain.
According to the obtained results from the in vitro tests with the lactic acid bacteria Lactobacillus plantarum GLB18, the strain could be considered as potential probiotics, according to their ability to survive in conditions of different pH - acid or alkaline environment.
Written on June 24, 2019, in Sofia, Bulgaria
Rositsa Tropcheva, PhD
Center for Applied Studies and Innovation