Cold plasma inactivation of Escherichia coli 0157:H7 biofilms

Authors: Niemira, Brendan; Boyd, Glenn; Sites, Joseph

Submitted to: Frontiers in Sustainable Food Systems
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/18/2018
Publication Date: 8/14/2018
Citation: Niemira, B.A., Boyd, G., Sites, J.E. 2018. Cold plasma inactivation of Escherichia coli 0157:H7 biofilms. Frontiers in Sustainable Food Systems. 2:47. https://doi.org/10.3389/fsufs.2018.00047.
DOI: https://doi.org/10.3389/fsufs.2018.00047

Interpretive Summary: Pathogens like Salmonella can persist in protected niches in food in processing environments. Cross-contamination of fresh produce and other foods from these pathogen reservoirs is a known risk factor. Industry requires a rapid, waterless, zero-contact, chemical-free method for removing pathogens from food-contact surfaces. Cold plasma (a type of energized gas) was tested for its ability to inactivate firmly attached cultures of Escherichia coli O157:H7 on a test surface (glass slides). These slides were placed on a conveyor belt and passed under a cold plasma emitter at various line speeds to provide exposure times of 5, 10 or 15 seconds. The test plate was positioned either 5 cm or 7.5 cm under a plasma jet emitter. The cold plasma was generated at one atmosphere using filtered air as the feed gas. The frequency of high voltage electricity used for the cold plasma was varied from 23 kHz to 48 kHz. At the closer spacing (5 cm), E. coli O157:H7 recovered from cold plasma treated biofilms was reduced by up to 96.1%, (5 seconds), 99.7% (10 seconds) and 99.95% (15 seconds). Increasing the distance to 7.5 cm generally reduced the maximum efficacy of the 5 and 10 second treatments, but had little effect on the 15 second treatment. Variation of the high voltage electricity had a greater effect on 10 and 15 second treatments, particularly at the 5 cm spacing. For each combination of time, distance and frequency, the firmly attached E. coli O157:H7 cultures all responded to cold plasma treatment in a manner consistent with each other, regardless of how long those cultures had been grown. Infra-red imaging showed that the antimicrobial effects were not the result of heat. The results show that cold plasma quickly and effectively inactivated a firmly attached form of E. coli O157:H7 contamination on a model food contact surface. This suggests that cold plasma is a possible tool for rapid disinfection of hard-to-sanitize materials associated with food processing.

Technical Abstract: Biofilms of Escherichia coli O157:H7 were grown for 24, 48 or 72 h on glass slides and exposed to atmospheric cold plasma, 23 kHz to 48 kHz, for 5, 10 or 15 s. Distance from emitter to biofilms was 5 cm or 7.5 cm. Cold plasma at 5 cm reduced biofilms by up to 1.41, 2.57 or 3.29 log cfu ml-1 for 5, 10 or 15 s, respectively. Cold plasma at 7.5 cm had reduced maximal efficacy at 5 s and 10 s (0.96 and 2.13 log cfu ml-1, respectively), but was unchanged for 15 s (3.29 log cfu ml-1). Biofilm age was not significant. All treatments were non-thermal. Cold plasma can be a sanitizing tool for food contact surfaces.