Evaluation of in vitro synergy between Tanreqing injection and antibiotics against methicillin- resistant Staphylococcus aureus

In light of the emergence and spread of antibiotic resistance and lack of new drugs, herbal drug and antibiotic combination may represent an interesting therapy option for treat multidrug resistant bacteria. The Tanreqing injection is Chinese herbal preparation made from Scutellariae radix (Huang Qin), Ursi fel (Xiong Dan), Naemorhedi cornu (Shan Yang Jiao), Lonicerae flos (Jin Yin Hua) and Forsythiae fructus (Lian Qiao). According to TCM, this formula clears heat and removes phlegm and it is used in China to treat respiratory tract infections (1). The present study was focused on the in vitro antibacterial activity and possible synergistic interactions of Tanreqing injection and selected antibiotics against methicillin-resistant Staphylococcus aureus (MRSA). The minimum inhibitory concentrations of Tanreqing injection, vancomycin and linezolid were determined by the broth microdilution method (16.5, 0.005 and 0.005 mg/mL, respectively) and synergy was studied by chequerboard technique and time kill curve assay. Obtained values of fractional inhibitory concentration index suggested no interactive effects of the tested combinations of Tanreqing injection and antibiotics. However, the time-kill experiments which are more informative giving an insight into both the rate and extend of antibacterial activity showed that the antibacterial efficacy of vancomycin and linezolid against MRSA was increased through their combination with Tanreqing injection. Combined application of 1/8 MIC vancomycin with 1/4 MIC Tanreqing or 1/8 MIC linezolid with 1/4 MIC Tanreqing revealed the ΔLC24 value of 2.63, suggesting synergistic effects of the tested TCM formula and antibiotics.