Extracellular antimicrobial activity of thermophilic lactic acid bacteria isolated from traditional fermented milk in Moldova

The aim of this study was to screen autochthonous thermophilic lactic acid bacteria isolates from traditional fermented milk in Moldova for extracellular antimicrobial activity against Escherichia coli and Listeria monocytogenes. Cell-free supernatants showed reproducible inhibition zones of 6.6–7.3 mm against E. coli and 5.5–6.3 mm against L. monocytogenes in the agar diffusion assay. Activity persisted after pH neutralisation and changed only slightly after catalase treatment, indicating that organic acids and hydrogen peroxide were not the main inhibitory factors. Protease treatment significantly reduced inhibition to 4.0–4.6 mm for E. coli and 3.0–3.5 mm for L. monocytogenes, although residual activity remained detectable (p < 0.05). These findings support a multifactorial inhibitory mechanism involving a proteinaceous (bacteriocin-like) fraction together with non-protein antimicrobial metabolites. The studied isolates are promising candidates for further purification and chemical characterisation of extracellular inhibitory compounds for food biopreservation.