[Comparative effect of beta-lactam activity on Pseudomonas aeruginosa as a function of resistance phenotypes]

In a multicentre study of the distribution in France of constitutive beta-lactamases produced by Ps. aeruginosa, the activities of 12 antibiotics were investigated comparatively according to resistance phenotypes. Antibiotics with the best activity, on a weight-for-weight basis, against "wild" non carboxypenicillin-resistant strains were ureidopenicillins among penicillins, ceftazidime and cefsulodin among cephalosporins and a new beta-lactam compound: N-formimidoyl-thienamycin. Thirty per cent of carboxypenicillin-resistant strains do not possess constitutive beta-lactamases. With most antibiotics, except N-f-thienamycin, minimum inhibitory concentrations (MICs) against these strains were increased 2 or 3-fold. However, the MICs of ureidopenicillins remained well below critical concentrations. The activities of the penicillins tested and of cefoperazone and cefsulodin against strains with constitutive beta-lactamases were considerably reduced. Those of other cephalosporins, such as cefotaxime, ceftriaxone, moxalactam, ceftazidime and azthreonam were little modified. However, in view of the plasma concentrations obtained, with the exception of ceftazidime and azthreonam the MICs of these cephalosporins were too high for satisfactory therapeutic results to be expected. Ps. aeruginosa strains producing beta-lactamases of the chromosomal cephalosporinase type are still rare (0.9%), but their number seems to be increasing. These strains, probably selected by treatments, are resistant to all beta-lactam antibiotics, and their degree of resistance to cephalosporins, ceftazidime included, is particularly high.