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 50μM) and low-frequency ultrasound (20kHz, 0.86-0.90WmL^-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 4min followed by exposure to 25μM 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.