In vitro evaluation of certain probiotic properties of lactic acid bacteria isolates

Abstract

Bacteria carrying genes responsible for antibiotic resistance cannot be used in food production. For this reason,exploring the antibiotic resistance profile of probiotic candidates and the antimicrobial substances theyproduce are essential for probiotic strain selection. The aim of this study was to develop and evaluate additionalelements of a complex in vitro test system for rapid and efficient selection of a large number of putativeprobiotic isolates. In a previous work, we had tested bacterial strains (n=217) isolated from Transylvanian rawsheep milk, cultured sheep milk, and sheep cheese samples and we reduced the sample number to a totalof six Gram-positive, non-hemolytic, catalase-negative, well-aggregating, good acid and bile acid toleratingstrains. In this research, we investigated the antibiotic resistance and antimicrobial production capacity of thepre-selected strains (n=6). The antimicrobial activity of the isolates was determined by the agar well diffusionassay. Strains E15, E66, E173, E198, and E216 were found to inhibit the growth of both Salmonella EnteridisATCC 13076 and the control strain (i.e., Lactobacillus acidophilus ATCC 4356). Antibiotic resistance testswere performed by the agar disk diffusion method. All six isolates belonging to the species of Levilactobacillusbrevis and Lactiplantibacillus plantarum were found to be resistant to several antibiotics and, therefore,cannot be used for the manufacture of commercial probiotic products. In conclusion, our in vitro test systemproved to be capable of effectively screening out unsafe isolates.