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MIC is the lowest concentration of an antibiotic required to inhibit the growth of an organism.

MIC was done against six conventional antibiotics e.g., Amoxicillin, Ceftriaxone, Gentamycin, Tetracycline, Ciprofloxacin, and Levofloxacin. E. coli O157:H7 was collected from hospital. The antibacterial activity of six conventional antibiotics was assessed against E. coli O157:H7by using the broth microdilution method then the fractional inhibitory concentration (FIC) index was used to define the interactions between antibiotics.

The minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) of antibiotics against E. coli O157:H7 showed that the highest concentration appeared in ciprofloxacin was 62.5 µg/ml. while ceftriaxone and gentamycin were 15.62 µg/ml. amoxicillin and levofloxacin was 7.812 µg/ml. the lowest concentration appeared in Tetracycline was 0.976 µg/ml. The higher FICI was seen in Ceftriaxone + Levofloxacin combination 0.3 % followed by Levofloxacin + Ciprofloxacin (0.1).

The antibacterial activity of both Ceftriaxone and Levofloxacin was enhanced by the combination which proved a highly synergistic effect against E. coli O157:H7.