THE COMBINED INHIBITORY EFFECT OF LYSOZYME AND LOW PH ON GROWTH OF LISTERIA-MONOCYTOGENES

Egg white lysozyme significantly inhibited growth of Listeria monocytogenes Scott A in tryptic soy broth (TSB) at 5-degrees-C and 25-degrees-C. The inhibition, which was mainly seen as a prolonged lag phase, was enhanced when the pH was lowered from 7.2 to 5.5. Lowering of the pH from 7.2 to 5.5 caused a lag phase of nine days at 5-degrees-C and addition of lysozyme (10,000 mu/ml) further increased the lag phase from 60 to 70 days. Similarly, lag phases at 25-degrees-C were 4 h (pH 5.5, no lysozyme) and 37 h (pH 5.5, 50,000 mu/ml lysozyme). Lowering of the pH from 7.2 to 5.5 did not, however, affect the lytic activity of the egg white lysozyme as measured by the decrease of absorbance of a suspension of live L. monocytogenes cells. The observed interactive inhibitory effect of lysozyme and low pH on growth is, therefore, suggested to be due to the growth retarding effect of low pH allowing enzymatic hydrolysis to exceed the rate of cell proliferation. The study shows that lysozyme may hold great potential for inhibition of L. monocytogenes in foods.