ANTIMICROBIAL EFFECTS OF ANTIBIOTICS IN COMBINATION WITH OREGANO ESSENTIAL OIL AGAINST ESCHERICHIA COLI AND KLEBSIELLA PNEUMONIAE

Essential oils from representatives of the family Lamiaceae have proven antimicrobial activity. There is solid scientific evidence for some of them that they have the potential for synergistic interactions with antibiotic agents widely used in clinical practice. The present study aims to investigate the interaction effects of oregano essential oil (OEO) with some conventional antibiotics against Escherichia coli and Klebsiella pneumoniae tested by agar methods. For this purpose, we selected Escherichia coli ATCC25922, Klebsiella pneumoniae ATCC13883, and four clinical isolates representing these species. OEO in a concentration of 2.5% (v/v) was combined with drugs proposed by EUCAST, 2024 (ampicillin, ceftriaxone, meropenem, gentamicin, ciprofloxacin) and tetracycline. The essential oil was analyzed using gas chromatography coupled with mass spectrometry (GC-MS). The analysis of the Oregano essential oil revealed the main component to be carvacrol (81,20%). The results from our study showed that the combinations of OEO with ceftriaxone and meropenem against Escherichia coli and Klebsiella pneumoniae and the combinations of the OEO with gentamycin and ampicillin against Escherichia coli resulted in an increase in the diameter of the zone of inhibition between 1 and 14 mm. Gentamicin and ciprofloxacin combined with OEO did not increase the zone of inhibition in most of the Klebsiella pneumoniae test samples. While examining the effect of tetracycline combined with OEO, we did not obtain unequivocal results for strengthening the antimicrobial effects against the microbes tested.