[Influence of antibiotic disk charge on concordance curve of cefotaxime ].

The activity of new cephalosporins is considerably higher than that of the first molecules. However antibiotic desk charge used for the agar diffusion method is still 30 micrograms. The disk diffusion - MIC distribution is not linear. The authors observed the influence of disk charge on the aspect of this distribution in order to determine the change permitting the best linearity. One hundred sixty five strains were studied using the agar diffusion and dilution methods. Their MICs were distributed from 0.015 to 128 mg/l. The disk charges studied were 5, 10, 15, 20, 25 and 30 micrograms. The results confirmed the parabolic aspect of concordance curve. This phenomenon was observed at higher disk charges (20, 25 and 30 micrograms). The linearity is better with the curves calculated from 5 and 10 micrograms. The correlation coefficients between these two charges were 0.92. However the inhibition diameters in 5 micrograms were too small to permit a clear differentiation between resistant and intermediate strains. The best discrimination is obtained with 10 micrograms. The zone diameter breakpoints were greater than or equal to 18 mm for susceptible 12 to 17 mm for intermediate, and less than or equal to 11 mm for resistant.