In Vitro Synergy of Biochanin A and Ciprofloxacin against Clinical Isolates of Staphylococcus aureus

Abstract

Many clinical isolates of Staphylococcus aureus (S. aureus) are resistant to numerous antimicrobials, including the fluoroquinolones (FQs). Flavonoids such as biochanin A (BCA) are compounds that are naturally present in fruits, vegetables, and plant-derived beverages. The goal of this investigation was to study the possible synergy between the antimicrobial agents BCA and ciprofloxacin (CPFX) when used in combination; CPFX was chosen as a representative FQ compound. We used S. aureus strain ATCC 25923 and 11 fluoroquinolone (FQ)-resistant methicillin-resistant S. aureus (MRSA) strains. Results from the drug susceptibility testing and checkerboard assays show that the minimum inhibitory concentration (MIC) of BCA ranged from 64 µg/mL to 512 µg/mL. When BCA was combined with CPFX, the fractional inhibitory concentration index (FICI) data showed that there was synergy in all 12 of the S. aureus strains tested. No antagonistic activity was observed in any of the strains tested. The results of time-kill tests and agar diffusion tests confirm that there was synergy between BCA and CPFX against S. aureus strains. These results suggest that BCA can be combined with FQs to produce a powerful antimicrobial agent.

Figure 1

Chemical formula of Biochanin A.

Figure 2

Time-kill curves of BCA and CPFX alone and in combination against clinical Methicillin -resistant S. aureus isolate 1662. The strain at starting inocula of 10⁶ CFU/mL were exposed to in vivo-achievable concentrations of 32 µg/mL BCA alone and in combination with 64 µg/mL CPFX. At 0, 3, 6, 24, and 48 h, aliquots were removed from each test tube to examine the cell metabolism ability. Symbols: , no drug control; □, CPFX; ▲, BCA; *, CPFX plus BCA.

Figure 3

Agar disk diffusion assay with various concentrations of BCA alone (A) or in combination with CPFX (B) against S. aureus isolate MRSA 1662. Lables: A1, CONTROL; A2, BCA 32 µg/mL; A3, BCA 64 µg/mL; A4, BCA 128 µg/mL; A5, BCA 256 µg/mL; B1, CPFX 128 µg/mL; B2, CPFX 128 µg/mL + BCA 16 µg/mL; B3, CPFX 128 µg/mL + BCA 32 µg/mL; B4, CPFX 128 µg/mL + BCA 64 µg/mL; B5, CPFX 128 µg/mL + BCA 128 µg/mL; B6, CPFX 128 µg/mL + BCA 256 µg/mL.