[Pseudomonas aeruginosa: acquired in vitro resistance to beta-lactams].

The development of resistance acquired in vitro to 5 semi-synthetic penicillins: carbenicillin, ticarcillin, azlocillin, mezlocillin, piperacillin and to 5 cephalosporins: cefotaxime, cefoperazone, moxalactam, cefsulodin and ceftazidime, was compared in 6 strains of Pseudomonas aeruginosa. The strains were selected on the basis of their phenotypic resistance: 3 were susceptible to all the betalactam antibiotics tested, 1 was resistant to carbenicillin and ticarcillin but remained susceptible to the others, 2 were susceptible to cefsulodin and ceftazidime but resistant to the others. The development of resistance was investigated by subsequent passages in liquid medium: up to 15 passages or up to an MIC of 4 096 mg/l for a penicillin or 512 mg/l for the cephalosporins. Irrespectively of the phenotypic resistance, for all cephalosorins the MIC became greater than or equal to 64 mg/l and very often greater than or equal to 512 mg/l after 1 to 3 passages. For the penicillin susceptible strains very high MICs were obtained more rapidly with azlocillin and piperacillin (1-2 passages) than with carbenicillin or ticarcillin (5-9 passages). These results are not in favour of a monotherapy with betalactam antibiotics, especially cephalosporins, in the treatment of Pseudomonas aeruginosa infections, but suggest the preferential use of carbenicillin and especially ticarcillin for sensitive isolates.