Investigation of vitality, antibacterial properties, and antagonistic effects of probiotic bacteria in probiotic dairy products

Probiotics play a significant role in the diet, and their contribution to the immune system has been recognised. Their effects on the gastrointestinal system have been evaluated for decades, and the mechanisms of the effects may differ. The aim of the present work was (i) to observe the changes in pH and bacterial counts in common probiotic dairy products, (ii) to isolate probiotic bacteria, (iii) to evaluate antibacterial resistance, and (iv) to evaluate their metabolites' antibacterial effects against common foodborne pathogens. To this end, 20 dairy products labelled "probiotics included" were collected. Isolation and enumeration of Lactobacillus spp., L. acidophilus, and Bifidobacterium spp. were carried out using de Man-Rogosa-Sharp agar (MRS), clindamycin/ciprofloxacin-included MRS agar (MRS-CC), and mupirocin (MUP) supplemented Bifidobacterium selective count agar (BSC-MUP), respectively. Isolates were identified using MALDI-TOF MS analyses, enumerated, and evaluated for their pH values at 1 to 28 d after production, at 1-w intervals. Selected isolates were analysed for antibacterial resistance using the disc diffusion method. Supernatants were then collected from selected probiotics grown in broth, and studied for their antagonistic effects against pathogens using disc diffusion and agar-well diffusion tests. IBM SPSS software was used for statistical analyses. Tests of normality and non-parametric analyses were performed. On the last day of analyses, 75% of the products met the probiotic bacteria vitality requirement of 106 CFU/g. Statistical analyses showed no correlation between increased acidity and bacterial decrease (p > 0.05), while the decrease in pH and bacterial count had significant relationship (p < 0.05). All selected isolates of probiotic bacteria (n = 10) showed multi-drug resistance (MDR) to 10 different common antibiotics. Antagonistic effects were present but weak (inhibition zones were 0 - 4 mm in diameter). When consumed in sufficient amounts, probiotics may inhibit possible pathogen growth in the gut microbiota via metabolites.