Enhanced antimicrobial effect of ultrasound by the food colorant Erythrosin B

The synergistic combination of the food colorant Erythrosin B (E-B, FD & C 3) (0, 25, and 50 mu M) and low-frequency ultrasound (20 kHz, 0.86-0.90 W mL(-1)) was evaluated against Listeria innocua. Although E-B was antibacterial by itself, the inactivation rate significantly increased in a concentration-dependent manner upon exposure to ultrasound and followed a sigmoidal behavior. The enhanced antimicrobial effect of E-B in the presence of ultrasound can be explained in part from a microbubble disappearance study in which it was confirmed that the presence of E-B enhances inertial cavitation, thereby enhancing the antimicrobial effect of ultrasound. The inactivation rate in a sequential treatment, where L. innocua was sonicated for 4 min followed by exposure to 25 mu M Erythrosin B, was comparable to that obtained by the simultaneous treatment, indicating complementary mechanisms of inactivation. Fluorescence microscopy showed attachment of E-B to the cells, which may explain its intrinsic antimicrobial property. Other mechanism may include the confirmed decrease in the cavitation threshold of water by addition of E-B, resulting in more effective cavitation. The study offers a proof-of-concept of a novel approach to complement ultrasound treatment for enhanced microbial inactivation.