ORIGINAL ARTICLE Antibacterial activities of zinc oxide nanoparticles against Escherichia coli O157:H7 Y. Liu 1 , L. He 1 , A. Mustapha 1 , H. Li 2 , Z.Q. Hu 3 and M. Lin 1 1 Food Science Program, Division of Food Systems & Bioengineering, University of Missouri, Columbia, MO, USA 2 Department of Mechanical and Aerospace Engineering, University of Missouri, Columbia, MO, USA 3 Department of Civil and Environmental Engineering, University of Missouri, Columbia, MO, USA Introduction Enterohemorrhagic Escherichia coli (EHEC) O157:H7 is one of the most important foodborne pathogens to the food industry and has resulted in a large number of highly publicized and expensive recalls (Al-Qadiri et al. 2006). The illness caused by E. coli O157:H7 can lead to inflammation of the colon and gives rise to diarrhoea and abdominal pain with bloody stools (Al-Holy et al. 2006). Escherichia coli O157:H7 can survive in acidic foods and its infective dose is as low as 10–100 cells (Lin et al. 1996). Outbreaks owing to this foodborne pathogen have increased in recent years. Foods of various origins, including spinach, lettuce, radish sprouts, pea salad, cantaloupe, alfalfa, apple cider, mayonnaise, raw milk, undercooked ground beef and roast beef were implicated in illnesses and outbreaks caused by E. coli O157:H7 (Smith and Fratamico 2005). Despite tremendous efforts that have been placed on controlling and eliminating E. coli O157:H7 from foods, foodborne illnesses associated with various food products continue to be documented. For example, in late 1992 and early 1993 in the United States and Canada, under- cooked ground beef contaminated by E. coli O157:H7 led to an outbreak affecting 732 individuals, with 55 cases of hemolytic-uremic syndrome (HUS) and 4 deaths (Smith Keywords antibacterial activity, E. coli O157:H7, nanoparticles, Raman, zinc oxide. Correspondence Mengshi Lin, Food Science Program, Division of Food Systems & Bioengineering, University of Missouri, Columbia, MO 65211, USA. E-mail: linme@missouri.edu 2008 ⁄ 2133: received 12 December 2008, revised 1 February 2009 and accepted 19 February 2009 doi:10.1111/j.1365-2672.2009.04303.x Abstract Aims: To investigate antibacterial activities of zinc oxide nanoparticles (ZnO NP) and their mode of action against an important foodborne pathogen, Escherichia coli O157:H7. Methods and Results: ZnO NP with sizes of 70 nm and concentrations of 0, 3, 6 and 12 mmol l )1 and NP-free solutions were used in antimicrobial tests against E. coli O157:H7. ZnO NP showed increasing inhibitory effects on the growth of E. coli O157:H7 as the concentrations of ZnO NP increased. A com- plete inhibition of microbial growth was achieved at the concentration level of 12 mmol l )1 or higher. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), and Raman spectroscopy were used to characterize the changes of morphology and cellular compositions of bacterial cells treated with ZnO NP and study the mode of action of ZnO NP against E. coli O157:H7. The intensity of lipid and protein bands in the Raman spectra of bacterial cells increased after exposure to ZnO NP, while no significant changes in nucleic acid bands were observed. Conclusions: ZnO NP were found to have antibacterial activity against E. coli O157:H7. The inhibitory effects increase as the concentration of ZnO NP increased. Results indicate that ZnO NP may distort and damage bacterial cell membrane, resulting in a leakage of intracellular contents and eventually the death of bacterial cells. Significance and Impact of the Study: These results suggest that ZnO NP could potentially be used as an effective antibacterial agent to protect agricultural and food safety. Journal of Applied Microbiology ISSN 1364-5072 ª 2009 The Authors Journal compilation ª 2009 The Society for Applied Microbiology, Journal of Applied Microbiology 107 (2009) 1193–1201 1193