Evaluation of probiotic and antibacterial properties of Lactobacillus fermentum SL163-4

Probiotics are live microorganisms, if consumed in enough quantity, they exert beneficial effects on human health owing to improvement of intestinal microbiota balance. In addition to the impact on gut microbiota, probiotics have important role on human physical and mental health. This matter demonstrates the increasing emphasis on the consumption of diet based on probiotics in order to treat and prevent of different chronic diseases, particularly those related to stress and inflammation cases. Lactic acid bacteria (LAB) are the most common strains used as probiotics. They are useful member of gut microbiota and belong to generally regarded as safe (GRAS) microorganisms. Because of the numerous benefits of LAB, the probiotic potential of different strains of this group of bacteria has been assessed broadly. Although various commercial species of probiotics are available in the market, determination of new strains with individual properties is noteworthy. Therefore, this research was aimed to investigate the probiotic and antimicrobial potential of Lactobacillus fermentum isolated from fermented food.  

In the study,the probiotic potential of Lactobacillus fermentum including its resistance to acid (pH 2.5, 3.5 and 5.5) and bile salts (0.2, 0.5, 0.8, 1.2 and 3%) was studied. To evaluate the bile salts resistance, 100 μl of prepared microbial suspension was cultured on MRS Agar media containing bile salts. Plates were incubated at 37 ᵒC for 24 hrs under anaerobic condition. After incubation period, the plates were inspected for bacterial colonies observed by naked eyes. The antimicrobial activity was measured using “Lawn on the spot” method against Listeria innocua, Staphylococcus aureus and Pseudomonas aeruginosa. The resistance of Lactobacillus fermentum was also assessed against commonly used antibiotic drugs (chloramphenicol, tetracycline, penicillin and gentamycin).  

Results shown that although Lactobacillus fermentum was not able to grow at pH 2.5, its viability in the pH 3.5 and 5.5 was 92 and 99%, respectively. This strain had also adequate resistance against different bile salt concentrations. In the present research, the growth rate of the examined strains was gradually reduced as the bile salt concentration was increased; so that  the higher and the lower growth rate was observed at 0.2 and 3% bile salt concentrations, respectively. Results shown that the tested Lactobacillus fermentum had acceptable bacteriostatic effect on the selected pathogenic bacteria. The inhibition zone diameter for Listeria innocua, Staphylococcus aureus and Pseudomonas aeruginosa was 12.6, 20 and 11.1 mm, respectively. The maximum diameter of inhibition zone was found on gram positive Staphylococcus aureus. Lactobacillus fermentum was susceptible to chloramphenicol, tetracycline and penicillin and was semi-resistant to gentamycin (comparison with table CLSI). Based on the obtained results in this study, it may be illustrated that Lactobacillus fermentum had capability to tolerate the lower pH and different bile salt concentrations. This strain showed the proper proficiency to inhibit pathogenic bacteria. Furthermore, it was susceptible to commonly used antibiotic drugs and therefore there is no concern about the transfer of antibiotic resistant gens into pathogenic bacteria. Consequently, this strain may be used as a probiotic and a natural preservative in production of functional food products.