en ميکروب شناسي Microbiology تحقيقي Original Paper <strong>Background and objectives:</strong> Drug resistance in <em>Staphylococcus aureus</em> and <em>Escherichia coli</em>, as severe pathogenic bacteria, has become a health challenge. However, nanoparticles have been introduced as effective candidates for their eradication. In this study, we investigated presence of genes involved in conferring resistance to silver nanoparticles in <em>S. aureus</em> and <em>E. coli</em> isolates and evaluated its association with minimal inhibitory concentration (MIC) of the nanoparticles against these isolates.<br> <strong>Methods</strong><strong>:</strong> The MIC of silver nanoparticles against 121 clinical isolates of <em>E. coli </em>and 183 <em>S. aureus </em>isolates was assessed by broth microdilution assay. Presence and expression of the silver resistance genes (<em>silE, silR/S</em>) in the isolates were investigated by PCR and real-time PCR, respectively.<br> <strong>Results</strong><strong>:</strong> The <em>silE </em>gene was found in three (1.6%) <em>S. aureus</em> and four (3%) <em>E. coli </em>isolates. MIC of silver nanoparticles against <em>S. aureus </em>isolates with the <em>silE</em> gene was 1, 2 and 8 &micro;g/ml. Moreover, the MIC of the nanoparticles against<em> silE</em>-positive <em>E. coli </em>isolates was 16 &mu;g/ml in three cases and 8 &mu;g/ml in one case. None of the <em>S. aureus </em>isolates contained the <em>silR</em>/<em>S </em>gene, but presence of both <em>silE and silR/S</em> was confirmed in two <em>E. coli </em>isolates. Real-time PCR showed no <em>sil</em> expression in the isolates containing the resistance genes.<br> <strong>Conclusion: </strong>The frequency of the silver resistance genes among <em>S. aureus </em>and <em>E. coli </em>isolates is very low. There is no relationship between presence of the resistance genes and the MIC value of silver nanoparticles. Staphylococcus aureus, Escherichia coli, Silver particles, MIC 7 12 http://mlj.goums.ac.ir/browse.php?a_code=A-10-856-2&slc_lang=en&sid=1 ezzat allah ghaemi eghaemi@yahoo.com 100319475328460017980 100319475328460017980 Yes Laboratory Science Research Center, Golestan University of Medical Sciences, Gorgan, Golestan, Iran fahimeh azadi fahimeh.azadi@ymail.com 100319475328460017981 100319475328460017981 No Department of Microbiology, School of Medicine, Golestan University of Medical Sciences, Gorgan, Golestan, Iran naeme javid naeme.javid@gmail.com 100319475328460017982 100319475328460017982 No Department of Microbiology, School of Medicine, Golestan University of Medical Sciences, Gorgan, Golestan, Iran hanieh bagheri bagheri.hanieh@ymail.com 100319475328460017983 100319475328460017983 No Department of Microbiology, School of Medicine, Golestan University of Medical Sciences, Gorgan, Golestan, Iran