Partial characterization of an antagonistic substance produced by Staphylococcus xylosus 1E and determination of the effectiveness of the producer strain to inhibit Listeria monocytogenes in Italian sausages

Staphylococcus xylosus 1E, isolated from Italian sausages, produced a substance with antagonistic activity against Listeria monocytogenes and other gram-positive bacteria during growth on a solid medium. Inhibitory activity in cell-free supernatant fluids of cultures in various broths was unsuccessfully researched. An active preparation of the inhibitory substance was obtained from the cells of the producer strain by desorption with 9 mol l(-1) urea. The inhibitor was not affected by autoclaving (121 degrees C for 15 min), whereas 50% activity remained after pronase and esterase treatment. SDS-PAGE allowed a diffuse smear of stainable material with inhibitory activity to he detected. The 1E inhibitor exhibited a bactericidal mode of action through adsorption to the cells and leakage of nucleic acids. The inhibition of L. monocytogenes by S. xylosus 1E was also observed in Naples-type sausages. After 21 days of maturation, in salami in which L. monocytogenes was challenged with S. xylosus 1E, the viable counts of the pathogen were reduced by approximately 2 log cycles, whereas 0.5 log reduction occurred in sausages with the pathogen alone. No L. monocytogenes were recovered after 75 days in sausages inoculated with S. xylosus 1E, while the pathogen was still present at this time in control sausages. (C) 1997 Academic Press Limited.