Flavonoids as antibiotic potentiators against Staphylococcus aureus planktonic cells and biofilms

Resistance to antibiotics is a global problem and occurs in part due to the overexpression of efflux pumps responsible for the removal of antibiotics from bacterial cells [1]. Since some flavonoids have been reportedly potent bacterial efflux pump inhibitors [2], we assessed quercetin (QUERC) and morin (MOR) for their combined activity with ciprofloxacin, tetracycline and erythromycin against the drug-resistant Staphylococcus aureus strains SA1199B, XU212 and RN4220, which overexpress the efflux pumps NorA, TetK and MrsA, respectively. The disc diffusion method was used. Since the formation of biofilms causes additional resistance, the combinations were also applied against biofilms of the strains grown in 96-well polystyrene microtiter plates at different conditions. The 24h biofilms obtained were analyzed regarding their culturability. SA1199B was loaded with ethidium bromide (EtBr) and the effect of the flavonoids on EtBr efflux efficiency was determined. Reserpine was used as a positive control. Additionally, the cytotoxicity of the flavonoids was evaluated in a mouse fibroblast cell line using the MTS assay. Despite showing no antibacterial activity, MOR and QUER (500 µg/mL) increased the activity of ciprofloxacin against SA1199B. The exposure of MOR for 1h against 24h biofilms allowed a high decrease in culturable cells for all strains. Also, both compounds had a relevant preventive activity on the formation of biofilms. QUERC presented a reduction of EtBr efflux similar to reserpine, proposing that it is involved in the inhibition of NorA. MOR had a low effect in reducing the efflux. Despite the potential cytotoxicity of QUERC and MOR at high concentration (IC₅₀ of 41.8 and 67.5 µg/ml, respectively), QUERC is commonly used as a supplement for several therapeutic purposes. This study emphasized the importance of co-therapies using flavonoids in order to promote more efficient treatments and decrease antimicrobial resistance to antibiotics.

Keywords: Flavonoids, co-therapy, efflux pump, potentiation, Staphylococcus aureus

References:

[1] Fernández L, Hancock REW. Adaptive and mutational resistance: role of porins and efflux pumps in drug resistance. Clin Microbiol Rev 2012; 25: 661 – 681.

[2] Abreu A, McBain AJ, Simões M. Plants as sources of new antimicrobials and resistance-modifying agents. Nat Prod Rep 2012, 29: 1007 – 1021.