Lactobacilli and gut pH: features of correlation in model experiments

Representatives of lactobacilli are among the most common probiotics, largely due to their prevalence as microorganisms involved in the fermentation of dairy products. However, research on their properties is often conducted under laboratory conditions, which are different from the conditions in the gut, making it difficult to extrapolate the obtained data. In our study, we demonstrate how the properties of two probiotic strains, Limosilactobacillus frumenti KL31 and Ligilactobacillus salivarius KL61, change under model conditions that approximate those of the gut, and we explore the relationship between lactic acid bacteria and the pH levels of the intestinal environment. Under gut-simulating conditions, lactobacilli produce less lactic acid and exhibit antioxidant and DNA-protective properties to a lesser extent than in laboratory conditions. The addition of simple or complex sugars stimulates a decrease in the pH of the model cecal contents. In turn, the pH level itself affects the growth of lactobacilli. The growth rate of Limosilactobacillus frumenti KL31 decreases when the pH of the medium increases above 7.0 since this microorganism prefers a lower pH. In contrast, Enterococcus spp. prefer a more alkaline pH, while the growth of Escherichia coli is largely independent of the environment's acidity. Due to the difficulty of assessing the probiotic properties of lactobacilli under standard laboratory conditions and the strong dependence of their growth and activity on intestinal conditions, it is necessary to use media and conditions that simulate the intestinal environment of the potential host organism when screening potential probiotic strains.