Antimicrobial activity of fusidic acid and disk diffusion susceptibility testing criteria for gram-positive cocci

The in vitro activity of fusidic acid was assessed and was compared with those of cloxacillin, cefamandole, vancomycin, teicoplanin, ofloxacin, ciprofloxacin, pefloxacin, and fleroxacin against 500 gram-positive cocci: 151 Staphylococcus aureus, 197 coagulase-negative staphylococci, and 152 Enterococcus faecalis strains. All clinical isolates were concomitantly tested by disk diffusion and agar dilution procedures as outlined by the National Committee for Clinical Laboratory Standards. The results with fusidic acid were further analyzed by regression line and error rate-bounded methods. With control American Type Culture Collection organisms, the values were within the limits of the National Committee for Clinical Laboratory Standards or published limits. The incidence of resistance to fusidic acid was 0.7% for S. aureus, 2.5% for coagulase-negative staphylococci, and 99.3% for E. faecalis. The correlation coefficient between the results of disk diffusion and agar dilution tests with fusidic acid was 0.90. Current interpretive criteria for susceptibility to fusidic acid (i.e., MIC of < 2 micrograms/ml and inhibitory zone of 20 mm) gave 1% false susceptibility (all strains being E. faecalis). This error rate is practically eliminated if a zone diameter of 21 mm is considered the breakpoint for susceptibility.