Discrepancies between results obtained by agar and broth techniques in testing of drug combinations

One hundred strains of Staphylococcus epidermidis were tested by the double-disk agar diffusion method for their susceptibility to cephalothin, to sulfamethoxazole-trimethoprim, and to combinations of these drugs. Forty-five strains showed a bridging inhibition zone between the two disks, suggesting a synergistic effect of the combination of cephalothin plus sulfamethoxozole-trimethoprim against these strains. Ten of these strains were retested against the individual drugs and their combination by the twofold broth dilution method. No strain showed higher susceptibility to any combination as compared with cephalothin alone.