Background: With respect to the increased antibiotic resistance specially E.Coli and Staph aureus, and the fact that the pattern of antibiotic resistance is variable in different regions, the present study was conducted to determine the effects of different methods of mutations on the antibiotic resistance patterns and plasmids in E. Coli and Staph. Aureus. Study was performed in Mahmoudieh Azad University Laboratory.

Materials and methods: This experimental study was performed on 6 strains of Staph. Aureus and E.coli isolated from skin wound, urinary infection and fecal sources. Antibiotic resistance was performed by disk method. Having induced mutation via UV, adding antibiotic gradually, and dilution of medium, the pattern of antibiotic resistance was determined. Finally, the presence or absence of plasmid in these bacteria were detected by electrophoresis.

Results: UV and adding antibiotic gradually induce antibiotic resistance. This also disappear number of plasmid bands in E Coli. Dilution of medium cause the bacterial colonies to expand and spread in semi solid medium, however, it did not induce any alteration in antibiotic resistance.

Conclusion: UV and adding antibiotic gradually induce antibiotic resistance and disappearance of plasmid bands in E Coli rather than Staph. Aureus. This may be due to structural differences in their cells wall. With respect to the increment in antibiotic resistance during the recent years, we strongly recommend to limit the usage of some antibiotics for only critical patients.