Increase in heat resistance of Listeria monocytogenes Scott A by sublethal heat shock

Log phase cells of Listeria monocytogenes Scott A were heat shocked in Trypticase Soy + 0.6% Yeast Extract broth at 40, 44, and 45°C for 3, 10 and 20 min at each temperature, followed by heating at 55â C for 50 minutes in order to determine an optimum heat shock response. Most heat shock temperatures significantly increased thermal resistance (p < 0.05). Increasing heat shock temperature and time allowed the organism to survive much longer at 50 to 65°C than nonheat shocked cells. The optimal heat shock condition was 45°C for 20 min where D-values at 55°C increased 2.3 fold in non-selective agar and 1.6 fold in selective agar in comparison to non-heat shocked cells. However, cells heat shocked at 48°C for 10 min gave more consistent results; these cells were heated at 50, 55, 60, and 65°C to determine a z-value. Although D-values notably increased due to heat shocking, z-values remained constant regardless of the plating medium.

When aerobically heat shocked cells (45°C for 10 min) were plated on a non-selective or a selective medium, a 1.4x increase in D-value was observed when enumerated under strictly anaerobic conditions. Aerobically heat shocked cells (48°C for 10 min) added to shrimp samples retained the increased heat resistance at 55°C when enumerated on a nonselective medium compared to the non-heat shocked cells. Heat shocking conditions may be created in pasteurization or minimal thermal processing of foods allowing increased heat resistance of pathogenic and spoilage microorganisms.