SYNERGISTIC EFFECTS OF RUTA GRAVEOLENS L. EXTRACTS WITH ERYTHROMYCIN AGAINST SKIN ISOLATES OF MLS-RESISTANT STAPHYLOCOCCI
Authors: Makevych N.V. , Kutsyk R.V. 
Published: July 2023

The aim: to research the synergistic interaction of garden ruta ethanolic extracts with erythromycin against skin isolates of MLS-resistant staphylococci.

Materials and methods. The study of the synergistic interaction of 5 water-ethanol extracts of the garden ruta herb (extractants - 40%, 50%, 70%, 90% and 96% ethanol) with erythromycin was performed on 11 clinical strains of skin isolates of MLS-resistant staphylococci. Determination of MLS-resistance was carried out by the disc-diffusion method. MIC of erythromycin of staphylococcal strains was determined by the method of two-fold serial dilutions in Muller-Hinton broth. A screening analysis of the potential ability for synergistic interaction of ruta herb extracts with erythromycin was carried out by the method of microdiffusion in agar. The validity of the synergistic interaction of the studied ruta herb extracts with erythromycin was confirmed by the сheckerboard assay with the calculation of the Fractional Inhibition Concentration Index (FICI).

Results. Using the qualitative method of microdiffusion in agar, it was established that subbacteriostatic concentrations of erythromycin increased the diameters of the zones of inhibition of the bacterial growth around the wells with all the studied extracts in 50±1.3% of the test strains. The best results were shown by the 96% extract of ruta herb, which increased the diameters of zones by 49.7-75.5%. Around the wells with 70% and 90% extracts, the diameters of zones increased by 32.4-48.3% and 34.6-52.4%, respectively. A decrease in MIC of erythromycin in the presence of efflux pump blockers - reserpine and potassium arsenate was observed in 2-4 times at four strains with R-phenotype, twice - at one strain with Neg-phenotype and one strain with D-phenotype of MLS-resistance.  90% and 96% ruta herb extracts showed a synergistic interaction with erythromycin among the largest number of researched strains, regardless phenotype of MLS-resistance (4-128-fold decrease in MIC of ERY). The least antibiotic potentiating ability was shown by 40% and 50% ruta herb extracts, under the influence of which the MIC of erythromycin decreased by 2-32 and 2-64 times, respectively.

Ruta herb extracts in 90% and 96% ethanol showed a synergistic interaction with erythromycin (average FICI values 0.49±0.42 and 0.42±0.25, respectively). Extracts in 40%, 50% and 70% ethanol were generally characterized by an additive effect (FICI values of 0.72±0.47, 0.63±0.24 and 0.68±0.38, respectively).

It should be noticed, that the synergistic interaction of erythromycin and subbacteriostatic concentrations (1/4, 1/8, 1/16 IPC) of ethanolic ruta herb extracts against the studied skin isolates of MLS-resistant staphylococci had a dose-dependent nature (F=6,9812; F>Fcrit.= 2,8916; p=0,000917).

Conclusions. Therefore, water-ethanol extracts of garden ruta herb demonstrate the ability to 4-128-fold reduction of MIC of erythromycin in MLS-resistant strains of S. epidermidis and S. aureus; extracts on 90% and 96% ethanol have more pronounced erythromycin-potentiating properties compared to extracts made on 40%, 50% and 70% ethanol. The synergistic interaction of erythromycin with ruta herb extracts was manifested on staphylococcal strains with efflux and combined mechanisms of MLS-resistance. The obtained experimental data suggest the presence of compounds in the garden ruta extracts which are capable of blocking the MrsA-mediated efflux of macrolides from staphylococcal cells.