Potentiation effects by usnic acid in combination with antibiotics on clinical multi-drug resistant isolates of methicillin-resistant Staphylococcus aureus (MRSA)

Abstract

The in vitro antibacterial activities of usnic acid (UA) in combination with six currently available antibiotics were evaluated through checkerboard microdilution and dynamic time-killing assays against Staphylococcus aureus and 10 clinical isolates of methicillin-resistant S. aureus (MRSA). The six antibiotics include three aminoglycosides (i.e., amikacin (AK), etimicin (EM), streptomycin (SM)), two glycopeptides (i.e., teicoplanin (TP), vancomycin (VA)) and a tetracycline (i.e., minocycline (MC)). UA alone showed MIC of 16 μg/mL against both S. aureus and MRSA strains. The checkerboard assay showed the range of fractional inhibitory indices (FICIs) as 0.156–1.500 against all the pathogens when UA was used in combination with the antibiotics. Significant bacteriostatic interactions of UA with TP and MC were observed. The enhancement of antibacterial activities against the tested pathogens were revealed by the bacteriostatic dose reduction indices (DRIs) ranges at 1–64 of UA and 1–32 of the antibiotics, especially the synergy of UA with TP by 90% and additive effects with VA by 50% isolates of MRSA strains, respectively. MC also showed 60% strains of synergy with UA. The time-killing curves further confirmed the bactericidal synergy among the combinations of UA with TP, AK, EM, and SM (1 × MIC, △LC24 = 3.406–4.344 log10CFU/mL) against one of the 10 MRSA strains, respectively. Other combinations showed additive effects or indifferences, while no antagonism occurred in all the tested combinations. The anti-MRSA potentiation is promising for further investigations in order to form a possible scenario of UA/antibiotics combinatory chemotherapy which would reduce their dosages and toxicological responses.