Effect of osmotic, alkaline, acid or thermal stresses on the growth and inhibition of Listeria monocytogenes

Five strains of Listeria monocytogenes (a, b, c, d and e) isolated from ind ustrial plants have been subjected to different osmotic, alkaline, acid or thermal stresses. The effects of these treatments on lag-phase (L) and grow th rate (mu) of cells in mid-log phase have been followed using an automate d optical density monitoring system. Increasing the osmotic pressure by the addition of different amounts of NaCl increased the lag phase and decrease d the growth rate. The same phenomena were observed after decreasing the pH of the medium to 5.8, 5.6 or 5.4 by addition of acetic, lactic or hydrochl oric acids. The inhibitory effect was: acetic acid > lactic acid > hydrochl oric acid. The addition of NaOH to attain pH values of 9.5, 10.0, 10.5 or 1 1.0 in the medium produced a dramatic increase of the lag phase at pH 10.5 and 11. Growth rates were also decreased while the maximal population incre ased with high pH values. These effects varied according to strains. Strain s d and e were the most resistant to acidic and alkaline stresses, and e wa s the most affected by the addition of NaCl. A cold shock of 30 min at 0 de grees C had limited effects on growth parameters. On the other hand, hypert hermal shocks (55 or 63 degrees C, 30 min) led to similar increased lag pha ses and to significant increases of the maximal population in all five stra ins.