Postbiotic-Based Extracts from Native Probiotic Strains: A Promising Strategy for Food Preservation and Antimicrobial Defense

<b>Background/Objectives</b>: The deterioration of food quality and safety is often linked to the presence of pathogenic and spoilage microorganisms. Postbiotics, including organic acids, enzymes, and bacteriocins produced by lactic acid bacteria (LAB), have emerged as promising next-generation food preservatives. This study investigates the biological and physicochemical properties of several postbiotic-based extracts (PBEs) comprising cell-free supernatant (CFS) and exopolysaccharide (EPS) fractions derived from three native probiotic strains: <i>Lactiplantibacillus plantarum</i> UTNGt2, <i>Lactococcus lactis</i> UTNGt28, and <i>Weissella cibaria</i> UTNGt21O. <b>Methods:</b> The antibacterial activity of these PBEs was assessed against multidrug-resistant <i>Escherichia coli</i> L1PEag1. Moreover, the antioxidant capacity and cytotoxicity along with the characterization of these formulations was assessed. <b>Results:</b> FU6 (CFS UTNGt28: EPS UTNGt2) and FU13 (CFS UTNGt21O) were found as the most potent formulations. Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) confirmed dose- and time-dependent damage to the bacterial membrane and cell wall. FU6 exhibited superior antioxidant activity and lacked hemolytic effects, whereas both FU6 and FU13 induced cell-specific responses in HEK293 (human kidney) and HT-29 (intestinal mucus-producing) cell lines. Furthermore, attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy identified characteristic absorption bands corresponding to proteins, lipids, carbohydrates, and nucleic acids, while proton nuclear magnetic resonance (¹H-NMR) spectroscopy revealed key monosaccharides, amino acids, and metabolites such as lactate and acetate within the extracts. <b>Conclusions:</b> FU6 and FU13 demonstrate potential as safe and effective postbiotic formulations at non-concentrated doses. However, further research is required to elucidate their molecular composition comprehensively and evaluate their applicability for broader and long-term use in food preservation and pharmaceutical development.