The <i>Campylobacter fetus</i> S layer provides resistance to photoactivated zinc oxide nanoparticles.

The antimicrobial activity of metal based compounds, including metal oxides, has resulted in numerous agricultural, industrial and medical applications. Zinc oxide nanoparticles are toxic to gram positive and gram negative bacteria as well as to some fungi. In this study we assess the sensitivity of <i>Campylobacter fetus</i>, a gram negative bacterial pathogen of humans and animals, to ZnO nanoparticles and determine whether the S layer protects <i>C. fetus</i> from the antibacterial action of these nanoparticles. Broth and agar dilution assays revealed that ZnO nanoparticles at 100 μg/mL were bacteriocidal for <i>C. fetus</i>. Resazurin reduction assays confirmed the absence of metabolic activity indicating that <i>C. fetus</i> cells had not entered into a viable but non culturable state. Photoactivation of ZnO nanoparticles greatly enhanced their antibacterial activity as evidenced by Minimum Bacteriocidal Concentration (MBC) values decreasing to 16 - 62.5 μg/mL as a function of strain. MBC assays completed in the presence and absence of catalase revealed that H₂O₂, a product of ZnO nanoparticle photoactivation, contributed to <i>C. fetus</i> but not to <i>C. jejuni</i> cell death. S layer expressing <i>C. fetus</i> strains were more resistant to H₂O₂mediated cell killing than were isogenic S layer deficient strains. This data indicates that <i>C. fetus</i> is sensitive to the antibacterial activity of ZnO nanoparticles and that the <i>C. fetus </i> S layer imparts protection against photoactivated nanoparticles.