Inactivation of Escherichia coli O157:H7 by High-Intensity Ultrasonication in the Presence of Salts

Inactivation of Escherichia coli O157:H7 suspended in salt solutions (Na(2)PO(4), NaCl, NaNO(3), NH(4)Cl, CaCl(2), and AlCl(3)) at concentrations ranging from 0 to 5% (w/v) by high-intensity ultrasound at two different temperature conditions (ice water bath, 40 degrees C water bath) and three ultrasonic intensity levels (9.5, 21.8, 49.2 W/cm(2)) 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. CaCl(2), NaCl, NaNO(3), Na(2)PO(4), and NH(4)Cl had little or no effect on survival of E. coli O157:H7 (<1-log reduction) regardless of other treatment conditions at 9.5 W/cm(2). Concentrations of >0.5% of CaCl(2), NaNO(3), Na(2)PO(4), and NH(4)Cl adversely affected survival (1.0- to 1.6-log reduction) with treatment of 21.8 W/cm(2) in an ice bath and showed greater inactivation (1.2-4.0-log reduction) in the 40 degrees C water bath. Treatment of 49.2 W/cm(2) showed the greatest impact regardless of other parameters. In the 40 degrees C water bath, treatment for 10 min at 49.22 W/cm(2) led to total inactivation for cells suspended in 0.5% NH(4)Cl, 1% CaCl(2), 2% NaCl, 5% Na(2)PO(4), and all concentrations of AlCl(3). Complete inactivation also occurred in an ice bath for 5% AlCl(3), which was shown to be the most effective salt. The most efficient treatment combination was in the 40 degrees C water bath for 10 min at 49.2 W/cm(2) with 5% AlCl(3). 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.