Antibacterial Activity Of Fusidic Acid Combined With Doxycycline Against Staphylococcus Aureus And Its Effect On The Mutant Selective Window

Objective: To investigate the effect of fusidic acid(FUS)combined with doxycycline(DOX)on the in vitro and in vivo antibacterial activity and the mutant prevention concentration(MPC)of Staphylococcus aureus.Methods: The minimum inhibitory concentration(MICs)of the two drugs alone and the MICs after the combined application against six strains of Staphylococcus aureus were determined by constant broth dilution method and micro-checkerboard dilution method respectively,and the fractional inhibitory concentration index(FICIs)were calculated.The colony of bacteria was determined by plate colony count method and the time sterilization curve were drawn when FUS and DOX was used alone and in combination.The MPCs of FUS and DOX alone and in combination of three strains of Staphylococcus aureus were measured by agar dilution method.A clinical Methicillin resistant Staphylococcus aureus(MRSA)model of mouse thigh infection was established,and the mice were divided into groups to explore the seven-day survival rate and the number of bacteria in the thigh muscle tissue after the treatment of FUS combined with DOX.Results: The FICIs of the combination of FUS and DOX for the standard strains ATCC29213,ATCC43300,clinical strains SA51,SA74,MRSA107 and MRSA212 were0.75,0.375,0.75,0.375,0.375 and 0.5,respectively.The combination of FUS and DOX showed additive anti-bacterial effect on ATCC29213 and SA51,and synergistic anti-bacterial effect on other strains.When the two drugs were used in combination at1/2MIC and 1MIC concentrations respectively,they showed additive anti-bacterial effect on ATCC29213 within 24 h,and the combination of the two drugs showed synergistic anti-bacterial and even bactericidal effect on ATCC43300 and MRSA107 within 24 h.The MPC of FUS alone to ATCC29213,ATCC43300 and MRSA107 were 256,>256,>256mg/L,respectively,and the MPC of DOX alone to the three strains were 8 mg/L.For ATCC29213,the combined use of FUS and DOX reduced the MPC of FUS from 256 mg/L to 16 mg/L,and reduced the MPC of DOX from 8 mg/L to 1 mg/L.For ATCC43300,the combined use of the two drugs reduced the MPC of FUS from >256 mg/L to 32 mg/L,and the MPC of DOX decreased from 8 mg/L to 1 mg/L.For the clinical strain MRSA107,the combined use of the two drugs reduced the MPC of FUS from>256 mg/L to 64 mg/L,the MPC of DOX decreased from 8 mg/L to 1 mg/L.The survival rates of mice within seven days in the FUS group,DOX group and FUS+DOX group were 0% and 33% and 50%,respectively.The seven-day survival rate of mice in the FUS+DOX group was higher than that of FUS or DOX single-agent groups,and the number of bacteria in the mice thigh muscle tissue was significantly lower than that in the FUS or DOX single-agent groups.Conclusion: The combination of FUS and DOX demonstrated stronger antibacterial effect against the six Staphylococcus aureus strains in vitro.FUS combined with DOX lowered the MPC and narrowed or even closed the MSW of the three Staphylococcus aureus strainsn in vitro.Thereby,the combined therapy could prevent the generation of drug-resistant mutant of the three strains.The combined therapy of FUS and DOX has stronger antibacterial effect than single drug in MRSA107 infected mice.