Kefir-derived Lacticaseibacillus paracasei as a promising candidate against planktonic and biofilm cells of foodborne Escherichia coli and Klebsiella pneumoniae.

This study aimed to identify kefir-derived microorganisms, evaluate their probiotic potential to select the most promising strain, and assess the selected strain's antibacterial and antibiofilm activities against foodborne isolates, Escherichia coli, and Klebsiella pneumoniae. Kefir grains were cultivated in selective media, and isolates were identified using MALDI-TOF mass spectrometry. Their acid and NaCl tolerance, auto- and co-aggregation, and antimicrobial activity were assessed to identify the most effective probiotic strain. Subsequently, the selected strain was subjected to spot overlay tests, broth microdilution assays, and biofilm inhibition experiments against E. coli and K. pneumoniae isolates. Enterococcus durans, Leuconostoc pseudomesenteroides, Lacticaseibacillus paracasei, and Kluyveromyces marxianus were identified. Among these, L. paracasei (Lpk-01) demonstrated the best probiotic characteristics, with 50% survival at pH 5, tolerance to 2% NaCl, and high co-aggregation with K. pneumoniae (40%). It inhibited pathogens (inhibition zones > 14 mm), and its cell-free supernatant (Lpk-cf) reduced biofilm biomass by ∼50%, with effects minimized upon neutralization, suggesting the involvement of organic acids. These findings affirm the antimicrobial potential of L. paracasei, highlighting its application as a natural alternative against bacterial pathogens.