Inactivation of Listeria innocua by a combined treatment of low-frequency ultrasound and zinc oxide

- Combination of ZnO and ultrasound is used against Listeria innocua.
- First order inactivation rate of combined method was ZnO concentration dependent.
- Antibacterial mechanism is in part Reactive Oxygen Species-mediated.
- Scanning Electron Microscopy shows adsorption of ZnO on bacterial surface.
- Proposed method may be useful for sanitation of wash water in fresh produce industry.

Microbial contamination of fresh produce remains a food safety issue in the US, with recent outbreaks linked to alfalfa sprouts and packaged salads. Chlorine-based sanitizers widely used on produce have limited efficacy and pose health risks. This study determined efficacy of combined low-frequency ultrasound with zinc oxide (ZnO) in inactivating Listeria innocua. ∼ 6 log(CFU/mL) L. innocua were treated with either 20 or 40 mM ZnO and sonication (20 kHz, 43-45 W, 120 μm amplitude) at room temperature for 0-30 min. 40 mM ZnO and ultrasound resulted in >5 log CFU/mL L. innocua reduction within 8 min, while individual treatments caused <1 log CFU/mL reduction. The inactivation rate was ZnO concentration-dependent. l-histidine, a known quencher of hydroxyl radicals and singlet oxygen, significantly attenuated bacterial inactivation, suggesting a Reactive Oxygen Species (ROS)-mediated antimicrobial mechanism. ZnO nanoparticle size decreased from ∼250 nm to ∼100 nm after sonication, potentially allowing ZnO to more easily penetrate cell membranes, while scanning electron microscopy (SEM) images of L. innocua suspended in 40 mM ZnO imply ZnO adheres on the surface of bacteria. ZnO-bacterial surface interactions can cause cell membrane damage and even cell death. The proposed technology has potential for sanitizing fresh produce industry wash-water.