Effect of an essential oil of native Thymes on mutant Escherichia coli strain with moderate resistance to Ciprofloxacin

Ciprofloxacin is effective against a wide range of bacteria, particularly Enterobacteriaceae. However, studies have shown that the resistance of Escherichia coli to ciprofloxacin is enhanced. Thyme is one of the medicinal plants whose essential oil has anti-microbial effects. This study was aimed to investigate the antimicrobial properties of local Thyme essential oil alone and in combination with ciprofloxacin on Escherichia coli mutant strain with intermediate resistance. In this descriptive laboratory study, Thyme plants (10 samples) were collected from Lorestan Province, west of Iran during 2016. Theses samples belonged to T. eriocali species. Plant essential oil was extracted by distillation method with Clevenger equipment. The antimicrobial properties of local thyme were determined by measuring minimum inhibitory concentration (MIC) of it alone and in combination with ciprofloxacin using sequential dilution method (macrodilution and microdilution methods) in Escherichia coli strains. Minimum bactericidal concentration (MBC) was measured by cultivation method. The interaction between essential oil and ciprofloxacin was determined by calculation of fractional inhibitory concentration index (FICI). MIC of essential oil for wild type strain MG1655 and mutant strain RE6 was 8 and 10 µl/ml, respectively. MBC was equal to MIC. 0.4 µl/ml of essential oil decreased 45 fold the MIC of ciprofloxacin in mutant strain and produced synergistic interaction (FICI=0.06). Thyme essential oil in concentration less than its MIC in combination with ciprofloxacin via synergistic interaction reduces antibiotic MIC and antibiotic resistance.