Lactobacillus paracasei subsp. paracasei was isolated in our laboratory from breast-fed newborn faeces and identified phenotypically and genotypically. The strain was able to produce a bacteriocin-like substance active towards listerial strains (Listenia innocua CLIP 74915 and Listeria monocytogenes EGDe). The maximum production of the substance by producing strain was detected in the late logarithmic growth phase (14 h in MRS and 18 h in BHI broths). It displayed bactericidal mode of action with leakage of cellular content (K+ and ATP) leading to cell lysis as secondary effect. A reduction of about 5 log with 35.7 +/- 0.2% of cell lysis and of about 4 log with 82.0 +/- 0.3% of cell lysis were observed, respectively, in a phosphate buffer and BHI suspensions. This was further demonstrated by electron microscopy that showed severe modifications in the cell morphology with a concomitant lysis. (C) 2008 Swiss Society of Food Science and Technology.