[Which betalactam antibiotic use as a marker of multiresistance in Pseudomonas aeruginosa?].

The determination of an indicating antibiotic for multiresistance, as methicillin in staphylococci, can be useful for Pseudomonas aeruginosa. Until now, the majority of the hygienists used ticarcillin, ceftazidim or imipenem in their investigations as markers of multiresistance for this species. Piperacillin has never been proposed for this purpose. To evaluate this choice, 2098 non-repetitive P. aeruginosa strains collected from 15 teaching hospitals in 1997-1999 were analysed, for eight antibiotics (ticarcillin, piperacillin, ceftazidim, imipenem, tobramycin, amikacin, ciprofloxacin, fosfomycin) according (i) to the results of the minimal inhibiting concentrations obtained by dilution in Mueller-Hinton agar, (ii) to their susceptibility following the criteria of Comité de l'antibiogramme de la Société Française de Microbiologie and (iii) to the determination of the mechanisms of resistance to the beta-lactam antibiotics. The low rates of sensitivity to the beta-lactam antibiotics, aminoglycosides, ciprofloxacin and fosfomycin were more frequent for piperacillin-resistant strains than for ceftazidim-resistant ones. Resistance to the other beta-lactam antibiotics are poor markers of multiresistance. In the light of the presented data, piperacillin seems to be, among the beta-lactam antibiotics, the best candidate as a marker of multiresistance for P. aeruginosa, followed by ceftazidim. This multiresistance is mainly found in strains overproducing AmpC cephalosporinase or transferable beta-lactamases. These mechanisms are well detected by resistance to piperacillin.