A comparison of agar dilution, identification of beta-lactamases and disc diffusion methods for assessing the sensitivity to ticarcillin-clavulanic acid.

The in-vitro antibacterial activity of Timentin has been evaluated with a view to proposing valid criteria for sensitivity and resistance. Minimal inhibitory concentrations (MIC) were determined for 284 strains including Enterobacteriaceae, Pseudomonas aeruginosa, Acinetobacter spp., staphylococci and enterococci. The same strains were also used for the determination of inhibition zones around discs containing 75 micrograms ticarcillin and 10 micrograms clavulanic acid, and a straight regression line was drawn correlating MIC with inhibition zones. The bacterial population was subsequently grouped according to the diameters of inhibition zones of the antibiogram for 2427 strains of Enterobacteriaceae classified by phenotypes. The comparison of MICs of Timentin and ticarcillin for these strains shows that for penicillinase-producing strains, the Timentin MIC is lower than for ticarcillin whereas cephalosporinase-producing strains have equal MICs for the two. Finally strains that produce many different beta-lactamases are not affected by the presence of clavulanic acid. These results should make it possible to propose provisional zone diameters for determining sensitivity and resistance to Timentin.