[In vitro study of the effects of a ticarcillin-clavulanic acid combination on Pseudomonas aeruginosa as a function of resistant phenotypes].

Activity of ticarcillin combined with clavulanic acid against 203 P. aeruginosa strains was studied. 159 strains produced a constitutive beta-lactamase with resistance to ticarcillin (MIC less than 128 mg/l) as a result. Minimal inhibitory concentrations (MICs) were determined using the agar dilution method for ticarcillin alone and combined with 4 and 8 mg/l clavulanic acid. Addition of clavulanic acid failed to change the activity of ticarcillin on ticarcillin-susceptible stains and on strains producing a constitutive cephalosporinase. A synergic effect was demonstrated for PSE and TEM type beta-lactamase producers and for OXA type beta-lactamase producers (4 halving dilutions and 1 to 2 halving dilutions respectively). However, because of the levels of baseline MICs, the combination brought the MIC of ticarcillin below the cutoff level for a small percentage of PSE strains (6%), moderate percentage of OXA strains (17 to 50%) and significant percentage of TEM strains. These percentages were slightly higher with 8 mg than 4 mg clavulanic acid. Given current data on the prevalence of the various beta-lactamases among resistant strains now being isolated in France, the ticarcillin + clavulanic acid combination will enable to achieve an MIC less than 128 mg/l for 11% (with 4 mg) or 16% (with 8 mg) of resistant strains.