Probiotic properties, antioxidant potential, bile salts tolerance and antibiotic susceptibility assessment of Streptococcus thermophilus isolates

Streptococcus thermophilus was considered one of the probiotic bacterial strains found in milk, cheese, and yogurt that provided health benefits to humans. Due to their probiotic potential, S. thermophilus can be very helpful in replacing chemical preservatives. This study came out to investigate the probiotic activity of different ten S. thermophilus isolates obtained from raw cow milk in eastern Morocco. The antimicrobial tests against multidrug-resistant bacteria (MDR): Staphylococcus aureus, and Escherichia coli were evaluated using the disc diffusion technique. In addition, the antioxidant potential was tested using the DPPH free radical assay. The bile salt tolerance was evaluated using M17 broth containing variable amounts of bile salt (0% as control; 0.1% and 0.3% as tests) at 37 °C for 3, 6 and 24 hours. Then cell counts were enumerated after 48 hours of incubation on M17 agar. Antibiotic susceptibility of S. thermophilus isolates against 20 antibiotics from 8 families (Rifamycins, Aminosides, Phenicols, Beta-lactam, Glycopeptides, Clindamycin, Cotrimoxazole, and Cyclines) were tested following a 24-hour incubation period using the antibiotic discs method. The diameters of the inhibition zones results were then compared to the recommendations made by the Antibiogram Committee of the French Microbiology Society of 2021. The results showed remarkable in vitro inhibitory ability of the S. thermophilus isolates against the tested multidrug-resistant bacteria S. aureus, confirming their potentiality as antibacterial agents. The mixtures of NaOH, catalase and proteinase K showed no inhibition zones against all tested multidrug-resistant bacteria, suggesting the main antibacterial effect of all isolates based on acids, hydrogen peroxide, and/or proteins. For the antioxidant activity, the DPPH inhibition values varied between 18.97% and 44.44% for the bacterial isolates ST9 and ST5, respectively. Notably, S. thermophilus isolates were able to survive up to 6 h of 37°C incubation with a bile salt concentration of 0.1%, where ST5 was the most promising isolate in terms of antioxidant activity. All isolates of S. thermophilus exhibited sensibility to all tested antibiotics except for rifampicin and trimethoprim-sulfamethoxazole compared to the European Committee on Antimicrobial Susceptibility Testing