[Spread in hospital units of a Pseudomonas aeruginosa phenotype producing a beta-lactamase highly inducible by clavulanic acid in vitro].

Pseudomonas aeruginosa strains belonging to the serogroup O:11 exhibiting susceptibility to ticarcillin (TIC) and resistance to the ticarcillin-clavulanic acid (CA) combination were found in 19 inpatients over a 14 month period. Mean inhibition diameters obtained using the agar diffusion method were 21.75 mm around the TIC disks (75 micrograms) and 15.96 mm around the TIC+CA disks (75 + 10 micrograms). With control PaO:11 strains, these diameters were 25.27 and 25 mm, respectively. MIC for ticarcillin determined using the checkerboard method rose to 64 mg with CA levels of 16 mg/l or more. CA exhibited dose-dependent antagonism on TIC killing curves when TIC levels approximated the MIC; this effect was no longer present with higher TIC levels. In the crude bacterial extract, a betalactamase of the cephalosporinase type was detected in the absence of induction and increased threefold after exposure to cefoxitin (100 mg/l) and fourfold after exposure to CA (5 mg/l). All these PaO:11 strains exhibited the same antimicrobial resistance phenotype with decreased susceptibility to ureidopenicillins and resistance to aminoglycosides and fluoroquinolones. The induction of a chromosome-encoded cephalosporinase by CA proved useful as an epidemiologic marker. Nosocomial spread of this phenotype was likely the result of selection due to use of antimicrobials.