Antibacterial activity of hen egg white lysozyme against Listeria monocytogenes Scott A in foods

Egg white lysozyme killed or prevented growth of Listeria monocytogenes Scott A in several foods. Lysozyme was more active in vegetables than in animal-derived foods that we tested. For maximum activity in certain foods, EDTA was required in addition to lysozyme. Lysozyme with EDTA effectively killed inoculated populations of 10(4) L. monocytogenes per g in fresh corn, fresh green beans, shredded cabbage, shredded lettuce, and carrots during storage at 5 degrees C. Control incubations without lysozyme supported growth of L. moncytogenes to 10(6) to 10(7)/g. Lysozyme had less activity in animal-derived foods, including fresh pork sausage (bratwurst) and Camembert cheese. In bratwurst, lysozyme with EDTA prevented L. monocytogenes from growing for 2 to 3 weeks but did not kill significant numbers of cells and did not prevent eventual growth. The control sausages not containing lysozyme supported rapid and heavy growth, which indicated that lysozyme was bacteriostatic for 2 to 3 weeks in fresh pork sausage. We also prepared Camembert cheese containing 10(4) L. monocytogenes cells per g and investigated the changes during ripening in cheeses supplemented with lysozyme and EDTA. Cheeses with lysozyme by itself or together with EDTA reduced the L. monocytogenes population by aproximately 10-fold over the first 3 to 4 weeks of ripening. In the same period, the control cheese wheels without added lysozyme with and without chelator slowly started to grow and eventually reached 10(6) to 10(7) CFU/g after 55 days of ripening. In the lysozyme-containing cheeses, however, L. monocytogenes also grew after 3 to 4 weeks of curing, which indicated that lysozyme was initially bacteriostatic but later allowed growth under the experimental conditions. Assay of lysozyme showed that lysozyme was still active in Camembert; therefore, changes during ripening in the cheese or in the bacterium probably overcame the inhibitory activity of lysozyme. In conclusion, results of this study suggest that egg white lysozyme could be useful as a preservative to protect against L. monocytogenes contamination in several foods, especially those prepared from fresh vegetables.