Exometabolite-Based Antimicrobial Formulations from Lactic Acid Bacteria as a Multi-Target Strategy Against Multidrug-Resistant Escherichia coli.

Background/

Objectives

The global increase in multidrug-resistant (MDR) bacterial infections underscores the urgent need for effective and sustainable antimicrobial alternatives. This study investigates the antimicrobial activity of exometabolite-based formulations (ExAFs), derived from the cell-free supernatants (CFS) of native lactic acid bacteria (LAB) applied individually or in combination thereof, against MDR-Escherichia coli strain L1PEag1.

Methods

Fourteen ExAFs were screened for inhibitory activity using time-kill assays, and structural damage to bacterial cells was assessed via scanning and transmission electron microscopy (SEM/TEM). The most potent formulation was further characterized by liquid chromatography-tandem mass spectrometry (LC-MS/MS) employing a Sequential Windowed Acquisition of All Theoretical Fragment Ion Mass Spectra (SWATH) approach for untargeted metabolite profiling.

Results

Among the tested formulations, E10, comprising CFS from Weissella cibaria UTNGt21O, exhibited the strongest inhibitory activity (zone of inhibition: 17.12 ± 0.22 mm), followed by E1 (CFS from Lactiplantibacillus plantarum Gt28L and Lactiplantibacillus plantarum Gt2, 3:1 v/v) and E2 (Gt28L CFS + EPS from Gt2, 3:1 v/v). Time-kill assays demonstrated rapid, dose-dependent bactericidal activity: E1 and E10 achieved >98% reduction in viable counts within 2-3 h, at 1× MIC, while E2 sustained 98.24% inhibition over 18 h, at 0.25× MIC. SEM and TEM revealed pronounced ultrastructural damage, including membrane disruption, cytoplasmic condensation, and intracellular disintegration, consistent with a membrane-targeting mode of action. Metabolomic profiling of E10 identified 22 bioactive metabolites, including lincomycin, the proline-rich peptide Val-Leu-Pro-Val-Pro-Gln, multiple flavonoids, and loperamide. Several compounds shared structural similarity with ribosomally synthesized and post-translationally modified peptides (RiPPs), including lanthipeptides and lassopeptides, suggesting a multifaceted antimicrobial mechanism.

Conclusions

These findings position ExAFs, particularly E10, as promising, peptide-rich, bio-based antimicrobial candidates for food safety or therapeutic applications. The co-occurrence of RiPP analogs and secondary metabolites in the formulation suggests the potential for complementary or multi-modal bactericidal effects, positioning these compounds as promising eco-friendly alternatives for combating MDR pathogens.