Increasi̇ng the sensi̇ti̇vi̇ty of Staphylococcus aureus isolates to tetracycli̇ne and qui̇nolones wi̇th caffei̇c aci̇d and its effect on efflux pumps

Staphylococcus aureus (S. aureus) is a Gram-positive bacterium that colonizes the skin and nasopharynx; when it enters tissue or the bloodstream, it can cause serious infections. Shortly after the discovery of penicillin and its widespread use in the treatment of staphylococcal infections, strains producing penicillinase emerged. Resistance to methicillin, which was used subsequently, also developed rapidly, and MRSA strains became a significant health threat. Efflux pumps, which play a role in the development of resistance, reduce the intracellular concentration of antimicrobial agents by transporting them out of the cell via cell membrane-bound transport proteins, thereby reducing their effectiveness and contributing to the development of resistance. NorA and TetK pumps, which belong to the Major Facilitator Superfamily (MFS), play an important role in the development of resistance to fluoroquinolones and tetracyclines, respectively. These pumps use energy obtained from proton motive force to transport substrates across the membrane. The efflux of antimicrobial agents via this mechanism prevents them from reaching their target site, thereby enabling the bacterium to survive and develop resistance. In this thesis study, the antimicrobial effect of caffeic acid on planktonic cells in Staphylococcus isolates in combination with tetracycline and fluoroquinolone antibiotics and its effects on NorA/TetK efflux systems were investigated. First, the presence of mecA and nuc genes in the isolates was confirmed, and the minimum inhibitory concentrations (MIC) and minimum bactericidal concentrations (MBC) of the antibiotics were determined. The interactions between caffeic acid and antibiotics were evaluated using the checkerboard method to determine the fractional inhibitory concentration (FIC) indices. At 1/4 MIC levels, the effects of caffeic acid, ciprofloxacin, and tetracycline, both alone and in combination, on norB and tetA transcript levels were measured using the qPCR method to determine ΔΔCt values. The findings indicate that caffeic acid has the potential to enhance antibiotic efficacy through efflux pump inhibition and may serve as an alternative approach in the treatment of resistant strains.