Inactivation ofEscherichia coliO157:H7 by High-Intensity Ultrasonication in the Presence of Salts

Inactivation of Escherichia coli O157:H7 suspended in salt solutions (Na2PO4, NaCl, NaNO3, NH4Cl, CaCl2, and AlCl3) at concentrations ranging from 0 to 5% (w/v) by high-intensity ultrasound at two different temperature conditions (ice water bath, 40°C water bath) and three ultrasonic intensity levels (9.5, 21.8, 49.2 W/cm2) was determined. Increases in sonication treatment time, intensity, and temperature led to increased lethality of E. coli O157:H7. However, cell lethality, as a function of solute concentration, varied depending upon type of salt. CaCl2, NaCl, NaNO3, Na2PO4, and NH4Cl had little or no effect on survival of E. coli O157:H7 (<1-log reduction) regardless of other treatment conditions at 9.5 W/cm2. Concentrations of >0.5% of CaCl2, NaNO3, Na2PO4, and NH4Cl adversely affected survival (1.0- to 1.6-log reduction) with treatment of 21.8 W/cm2 in an ice bath and showed greater inactivation (1.2–4.0-log reduction) in the 40°C water bath. Treatment of 49.2 W/cm2 showed the greatest impact regardless of other parameters. In the 40°C water bath, treatment for 10 min at 49.22 W/cm2 led to total inactivation for cells suspended in 0.5% NH4Cl, 1% CaCl2, 2% NaCl, 5% Na2PO4, 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. The most efficient treatment combination was in the 40°C water bath for 10 min at 49.2 W/cm2 with 5% AlCl3. Inactivation of E. coli O157:H7 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.