Organic acid profile of cell-free supernatants from probiotic lactic acid bacteria strains

Background: Lactic acid bacteria (LAB) produce a wide range of metabolites, among which organic acids as the predominant end products of carbohydrate fermentation are considered major contributors to the antimicrobial activity of cell-free supernatants (CFS). However, the activity of LAB-derived CFS varies among different strains and the role of organic acids in acid-dependent antimicrobial activity remains poorly understood. Objective: The aim of this study was to characterise the organic acid profile, to analyse the relationship between pH and the total concentration of organic acids and to assess the proportion of their undissociated forms in the cell-free supernatants from probiotic LAB strains. Methods: 24-h cultures of five probiotic LAB strains were centrifuged at 2,500 g and supernatants were sterilised. Organic acids were quantified by capillary electrophoresis. Pearson’s correlation and linear regression were used to evaluate the relationship between the total organic acids concentration and pH. The proportion of undissociated lactic and acetic acids was calculated using the Henderson–Hasselbalch equation. Results: It was shown that lactic and acetic acids predominated in the CFS of all strains, whereas citric, oxalic, butyric, and valeric acids were detected at low or trace concentrations. The highest total concentration of organic acids was detected in the supernatant of Lbs. casei IMB 7412, whereas the lowest was observed in that of Lpb. plantarum A. A strong negative correlation between the total concentration of organic acids and the pH values of the CFS was demonstrated, indicating a decrease in pH as total organic acid concentration increases. The highest proportion of undissociated organic acids was calculated for the CFS of Lbs. casei IMB 7412, whereas the lowest for that of Lab. bulgaricus A6. Thus, among the studied strains Lbs. casei IMB 7412 was characterized by the highest total organic acid concentration, the highest L/A ratio, the lowest pH, and the greatest proportion of undissociated forms of both lactic and acetic acids. Conclusions: Comprehensive assessment of the organic acid profile can facilitate the comparative characterization of probiotic LAB strains and help better understand their potential pH-dependent antimicrobial properties.