Introduction Staphylococcus aureus is a major cause of skin and soft tissue infections, endocarditis, osteomyelitis, toxic shock syndrome (TSS), and the infections related to medical tools which could threaten human lives. 1,2 Human being is the main carrier of S. aureus. This asymptomatic bacterium is able to settle on the skin or inside the nasal cavity of humans and animals, to the extent that about 20% of humans are permanent carriers of this bacterium. 3 In some hospitals, this bacterium is more commonly found in the intensive care unit (ICU) which could be lethal. Due to increasing drug resistance, this bacterium has now turned into an important hygiene problem worldwide. 4,5 After observing the first case of methicillin- -resistant Staphylococcus aureus (MRSA) (1960s), mupirocin was the first topical drug used to treat MRSA skin infections in 1986. This drug is a unique antimicrobial agent in comparison with other antibiotics, which is originated from Pseudomonas fluorescens. 6 Mupirocin (pseudomonic acid A) has the capacity to interact with isoleucyl- tRNA synthetase (IRS) and block protein synthesis. 7 Due to its short half-life after being injected and high protein connections, this drug is currently used in the topical form. Unfortunately, the current indiscriminate use of drugs has developed a relative drug resistance in this bacterium and has created resistant species of this bacterium which hinder treatment of some diseases. Therefore, it is recommended to use various combinations or antimicrobial agents such as nanoparticles to stop or control such resistance. 8,9 Nanotechnology, a hotspot in science of materials, is used increasingly in researches and has applications related to human doi 10.15171/ijbsm.2018.14 Antibacterial Effect of Zinc Oxide Nanoparticles on Mupirocin-Resistant Staphylococcus aureus Isolated From Nasal Carriers Afsaneh Hajimohammad 1 , Leila Fozouni 1* 1 Department of Biology, Gorgan Branch, Islamic Azad University, Gorgan, Iran Abstract Introduction: Staphylococcus aureus is one of the main causes of hospital infections. The present study aimed to determine the antimicrobial effect of zinc oxide nanoparticles (ZnONPs) on the growth of mupirocin-resistant S. aureus isolates taken from hospital carriers. Methods: The samples were taken from the anterior nasal parts of 150 hospital staff in Gorgan, using a sterile swab. Being cultured on Mannitol salt agar, the suspected colonies were identified through gram staining as well as catalase, coagulase and DNase tests. Resistance of the strains to mupirocin was examined using microdilution broth test. The antibacterial effect of ZnONPs on the mupirocin-resistant strains was also investigated using agar well diffusion method. Results: In this study, 48 isolates (32%) were identified as S. aureus, out of which 3 isolates (6.2%) showed high resistance to mupirocin and 14 isolates (29.2%) showed low resistance to this antibiotic. The results of this study revealed that ZnONPs had the highest inhibitory effect on the growth of mupirocin-resistant S. aureus in the density of 400 mg/mL. Conclusion: The ZnONPs used in this study had a high dose-dependent antimicrobial activity against all drug-resistant S. aureus strains. Keywords: Staphylococcus aureus, Mupirocin, Zinc oxide nanoparticles, Hospital carriers *Correspondence to Leila Fozouni, Assistant Professor, Department of Biology, Gorgan Branch, Islamic Azad University, Gorgan, Iran/ Tel:+98-911-151-8674, Fax: +98-11- 3329496 Email: lili_kia@yahoo.com Received September 27, 2017 Accepted April 3, 2018 Published online June 30, 2018 Int J Basic Sci Med. 2018;3(2):78-82 Original Article http://ijbsm.zbmu.ac.ir/ © 2018 The Author(s); Published by Zabol University of Medical Sciences. This is an open-access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Please cite this article as follows: Hajimohammad A, Fozouni L. Antibacterial effect of zinc oxide nanoparticles on mupirocin-resistant Staphylococcus aureus isolated from nasal carriers. Int J Basic Sci Med. 2018;3(2):78- 82. doi:10.15171/ ijbms.2018.14.