THE EFFECT OF SUBLETHAL HEAT-SHOCK AND GROWTH ATMOSPHERE ON THE HEAT-RESISTANCE OF LISTERIA-MONOCYTOGENES SCOTT-A

Log phase cells of Listeria monocytogenes Scott A were heat shocked in Trypticase Soy + 0.6% yeast extract (TSYE) broth at 48-degrees-C for 10 min, followed by heating at 55-degrees-C for up to 50 min. Heat resistance was determined using nonselective (TSYE) and selective (McBride Listeria) enumeration media which were incubated under aerobic and anaerobic environments. D55-degrees-C-values for heat shocked cells were 2.1-fold higher than nonheat shocked cells (18.7 min vs. 8.89 min) when cells were enumerated on TSYE agar aerobically and 2.2-fold higher (26.4 min vs. 12.0 min) for cells enumerated anaerobically on TSYE agar. When cells were enumerated aerobically on McBride Listeria (ML) agar, D55-degrees-C-values for heat shocked cells were 1.4-fold higher than nonheat shocked cells (9.55 min vs. 6.69 min). No growth was observed on ML agar anaerobically. The physiological condition of the microorganism, the enumeration medium, and the growth environment greatly affected the heat resistance of log-phase cells of Listeria monocytogenes Scott A.