Comparative LC-MS profiling and antimicrobial evaluation of Kefir-derived and Lactobacillus-derived postbiotics

Kefir harbours a mixed community of lactic acid bacteria and yeasts, and the postbiotics obtained from it have attracted interest as natural functional ingredients with antimicrobial and health-promoting properties. It is not established, however, whether a postbiotic obtained directly from whole kefir grains differs from one produced by Lactobacillus strains isolated from the same grains. The objective of this study was therefore to compare the metabolite profile and the antimicrobial activity of a kefir-derived postbiotic (KP) and a Lactobacillus-derived postbiotic (LP). Cell-free preparations were obtained by controlled fermentation, centrifugation and membrane filtration. Their metabolite composition was characterized qualitatively by Liquid Chromatography–Mass Spectrometry (LC-MS), and their antimicrobial activity was determined by viable count reduction against Staphylococcus aureus ATCC43303 y Candida albicans ATCC10231 after 12 h of contact. Both preparations reduced the viable counts of both test organisms. KP reduced S. aureus by 2.99 ± 0.04 log10 CFU/mL and C. albicans by 1.74 ± 0.05 log10 CFU/mL, whereas LP reduced both organisms to below the detection limit of the plating method, corresponding to reductions of at least 3.02 ± 0.01 and 4.11 ± 0.01 log10 CFU/mL, respectively. The difference between the two preparations was statistically significant against C. albicans (P < 0.001) but not against S. aureus (P = 0.28), where both preparations approached the detection limit of the assay. The metabolite profiles of the two preparations largely overlapped: 18 of the annotated compounds formed a shared core that contained the principal antimicrobial organic and aromatic acids, while only a small number of features differentiated them, namely ascorbic acid and acetylated aromatic amino acids in KP, and free valine and short peptides in LP. Because the LC-MS characterization was qualitative and based on a single injection per preparation, and because no uninoculated- medium blank was analysed, these compositional differences should be regarded as preliminary. Within these limitations, the results suggest that the microbial source may influence both the metabolite profile of kefir postbiotics and, against C. albicans, their antimicrobial performance, and that both preparations merit further evaluation as natural antimicrobial agents for food applications.