EVALUATION OF THE SYNERGISTIC EFFECT OF PHOSPHOMYCIN AND CITRUS AURANTIFOLIA'S ESSENTIAL OIL ON METHICILIN RESISTANT STAPHYLOCOCCUS AUREUS ISOLATES

Staphylococcus aureus is a gram-positivecoccal bacteriumthat that can cause a range of infections. Methicillin resistance in these bacteria is often in companion with resistance to multiple antibiotics. High prevalence of these isolates can cause treatment failure. Phosphomycin is a peptidoglycan biosynthesis inhibitor that is used in treating infections caused by multi-drug resistant bacteria. The antimicrobial effects of Citrus aurantifolia’s essential oil on methicillin resistant S. aureus have been shown previously. The aim of this study was to evaluate the synergistic effect of sub-MBC doses of Phosphomycin with Citrus aurantifolia's essential oil against methicillin-resistant S. aureusisolates. In this study, 23 clinical isolates of methicillin-resistant S. aureus were collected from clinical samples and these isolates were identified based on biochemical methods. Susceptibility of isolates to different antibiotics was tested by disk diffusion method. In order to determine the minimum inhibitory and bactericidal concentrations of phosphomycin and Citrus aurantifolia's essential oil, serial dilutions of antibiotic and essential oil were prepared in broth medium in concentrations ranging between 0. 25-128mg/L and 0. 078 10٪-, respectively. The amounts of minimum bactericidal concentrations for a mixture of dilutions of essential oil with sub-MIC doses of Phosphomycin were also determined. Among 23 methicillin resistant isolates, 13% were sensitive to all investigated concentrations of phosphomycin and 87 % were resistant. The average amounts of minimum inhibitory and minimum bactericidal concentrations for the other isolates were 56 ± 52. 9 and 76. 11±42. 15 µ/ml, respectively. The average amounts of minimum bactericidal concentrations for the Citrus aurantifolia's essential oil was 3%±4%. Synergism has been shown in 77% of investigated isolates but not about other 23% of isolates. Due to the higher resistance of isolates tested in this study in comparison with others, it seems that there is a need for exact evaluation of susceptibility tests and being cautious in using of phosphomycin alone. Also regarding to the synergistic effect of phosphomycin and Citrus aurantifolia's essential oil on some methicillin resistant isolates, using the combination of them is advised as an effective antimicrobial agent.