Background and Aims: Staphylococcus aureus as a�Gram- positive coccus causes a variety of infections in humans. It is one of the infectious agents in hemodialysis patients. Those patients who carry this organism at their nose are exposed to infection and possible morbidity and mortality due to this bacterium. Resistance to antibiotics in staphylococci is increasing. Resistance development is due to mutation and by plasmid DNA transmission. The aim of this study was to determine plasmid profile and antibiotic resistance of Staphylococcus aureus strains isolated from nasal carriers in dialysis patients in Imam Khomeini Medical Center. Susceptibility testing to antibiotics, plasmid extraction and analysis and epidemiologic relationship of these isolates were investigated.�

  Methods: In this study nasal specimens of 107 patients in dialysis ward of Imam Khomeini Medical Center were collected and cultured on blood agar plates. The colonies were identified as S.aureus strains. The susceptibility of 50 strains isolated from the patients against 12 antibiotics were tested using Kirby- Bauer standard method. A standard S.aureus strain (ATCC29213) was used to control quality of antibiotic discs. The isolates were cultured on LB medium and plasmid DNAs were extracted and electrophoresed on agarose gel using Parisi et al method.

  Results : The results of resistance rate against 12 used antibiotics were as follows: resistance of the strains against gentamicin, oxacillin, neomycin, clindamycin, erythromycin, cotrimoxazole, choloramphenicole, tetracycline, and ciprofloxacin were 20%, 28%, 30%, 26%, 30%, 44%, 32%, 36%, and 10%, respectively. All of the strains were resistant to amoxycillin and penicillin and none of them were resistant to vancomycin. Of 50 S. aureus strains, only 27 strains contained plasmid DNA. Most of the strains revealed a big plasmid. Plasmid profiles of the strains will be presented.

  Discussion: Our results showed that there was a close relationship between high resistance to antibiotics and presence of plasmids in S. aureus strains. Similarities among resistance to antibiotics and plasmid profiles in our strains isolated from the same ward showed that these strains were from the same sources and indicated a unique clonal possibility. The resistance to antibiotics of the strains lacking plasmids could be from choromosomal resistance