In vitro synergy between sulfamethoxazole and trimethoprim on strains of Staphylococcus aureus sensitive and resistant to methicillin

: The synergy between sulfamethoxazole (SMZ) and trimethoprim (TMP) is studied on 91 methicillin sensitive S. aureus strains (methi.S) and 95 methicillin resistant S. aureus strains (methi.R). In methi.S strains, the MIC 90% of SMZ, TMP and SMZ + TMP (19 SMZ/1 TMP) is respectively 64, 0.5 and 2 mg/l. 98% of methi.S strains have a FIC index less than or equal to 0.6 and only one strain has a FIC index greater than or equal to 0.6. In methi.R strains, the MIC 90% of SMZ, TMP and SMZ + TMP is respectively 256, 0.5 and 4 mg/l. 77% of methi.R strains have a FIC index less than or equal to 0.6. The relationship between FIC index and disc diffusion method is studied: bridging inhibition zone between SMZ 200 mcg disc, and TMP 2.5 micrograms disc and difference of the inhibition diameter between SMZ + TMP disc and TMP 1.25 micrograms disc. There is a good correlation only in methi.S strains in the first method and in methi.S and methi.R strains in the second method.