Inactivation of Escherichia coli O157:H7 and Listeria monocytogenes by High-Intensity Ultrasound in the Presence of Salts

Inactivation of Escherichia coli O157:H7 and Listeria monocytogenes suspended in salt solutions Na2PO4, NaCl, NaNO3, NH4Cl, CaCl2, AlCl3, and L. monocytogenes in salt solutions NaCl, NaNO3, AlCl3 at concentrations ranging from 0 to 5% (wt/vol) by high-intensity ultrasound at two different temperature conditions (ice water bath, 40ºC water bath) and three ultrasonic intensity levels (9.49, 21.83, 49.22 W/cm2) was determined. Increases in sonication treatment time, intensity and temperature led to increased lethality of both pathogens. However, cell lethality, as a function of solute concentration, varied depending upon type of salt. CaCl2, NaCl, NaNO3, NaPO4, and NH4Cl had little or no effect on survival of E. coli O157:H7 (< 1 log reduction) regardless of other treatment conditions at 9.49 W/cm2. Concentrations > 0.5% of CaCl2, NaNO3, NaPO4, and NH4Cl adversely affected survival (1.0 - 1.6 log reduction) with treatment of 21.83 W/cm2 in an ice bath, and showed greater inactivation (1.2 - 4.0 log reduction) at 40°C. Treatment of 49.22 W/cm2 showed the greatest impact regardless of other parameters. At 40°C, treatment for 10 minutes at 49.22 W/cm2 led to total inactivation for bacteria suspended in 0.5% NH4Cl, 1% CaCl3, 2% NaCl, 5% NaPO4, and all concentrations of AlCl3. Complete inactivation also occurred in an ice bath for 5% AlCl3, which was shown to be the most effective salt.

NaCl and NaNO3 had little or no effect on survival of L. monocytogenes (< 1 log reduction) regardless of other treatment conditions at 9.49 W/cm2. Concentrations >0.5% of NaCl and AlCl₃ adversely affected survival (0.2 -1.6 log reduction) with treatment of 21.83 W/cm2 in an ice bath and showed greater inactivation (0.4 - 5.2 log reduction) at 40°C. Regardless of other treatment parameters, sonication at 49.22 W/cm2 showed the greatest lethal impact. Treatment for 10 minutes at 40°C at 49.22 W/cm2 led to a 4.8 - 5.7 log reduction for cells suspended in all concentrations of NaNO3. Typically, L. monocytogenes was more resistant to ultrasound treatment than E. coli O157:H7. Inactivation of pathogens is attributed to cavitation-induced shear forces, reaction of cavitation-generated hydrogen peroxide with microbial cell wall constituents and electrostatic interactions of dissociated salts with cell membranes.