Synergistic interactions of levofloxacin with quercetin against Staphylococcus aureus and Pseudomonas aeruginosa biofilm

Introduction. The increased use of fluoroquinolones in ophthalmology has led to increasing resistance of Staphylococcus aureus and Pseudomonas aeruginosa, the most common pathogens associated with chronic keratitis linked to biofilm. An effective alternative therapeutic strategy can be the combinations of antibiotic and phytochemical having a synergistic activity and potential implication in delaying resistance. The aim of this study was to assay combinations of levofloxacin with quercetin against resistant biofilm forming strains. Materials and Methods. Levofloxacin (LVF) and quercetin (QUE) alone and in combinations were tested against S. aureus and P. aeruginosa strains. Cytotoxic activity of QUE was evaluated by MTT method on human corneal epithelial (HCE) cells and by Artemia salina lethality bioassay. Antibacterial and antibiofilm activities were tested by microdilution techniques. The checkerboard assay was used to assess synergistic interactions between LVF and QUE both on planctonic growth and on biomass production of biofilm. Results. The MIC value of LVF was 0.122 μg/mL against S. aureus ATCC 6538 and S. aureus ATCC 43300 (MRSA) strains. The MIC values of antibiotic were 2 μg/mL and 250 μg/mL against P. aeruginosa ATCC 9027 and P. aeruginosa DSM 102273, respectively. QUE showed no activity against all strains at the maximum non-toxic concentration assayed. At ½ MIC LVF did not inhibit biofilm formation of all investigated strains, whereas QUE reduced biofilm biomass by 80% and 50% for S. aureus ATCC 6538 and MRSA, respectively. The combinations LVF with QUE showed synergistic interactions against planctonic cells of S. aureus ATCC 6538 with FIC index of 0.27 and a percentage of growth reduction of 63%, and of S. aureus ATCC 43300 with FIC index of 0.19 and growth reduction of 32%. The associations demonstrated additive interactions against P. aeruginosa ATCC 9027 and P. aeruginosa DSM 102273 with FIC index of 0.62 and 0.56, respectively. The combinations showed synergistic interactions against biofilm biomass of S. aureus strains with FIC index of 0.16, and of P. aeruginosa ATCC 9027 and P. aeruginosa DSM 102273 with FIC index of 0.37 and 0.07, respectively. Discussion and Conclusions. The synergistic results obtained demonstrated as the combinations of LVF with QUE improve the activity of LVF against the planctonic growth and biomass production of biofilm of investigated strains. Further studies are underway to evaluate if the obtained synergistic concentrations loaded in Self-Nanoemulsifying Drug Delivery Systems (SNEDDS) can further enhance LVF antibiofilm activity.