Simple method to determine β-lactam resistance phenotypes in Pseudomonas aeruginosa using the disc agar diffusion test

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Abstract
Background: Pseudomonas aeruginosa is a major opportunistic bacterial pathogen in nosocomial infections because of the increasing prevalence of resistance to many of the commonly used antibiotics. To ensure optimal efficiency of antibiotic treatment against this species, antibiotic susceptibility tests must be interpreted with caution. Most microbiologists now consider it essential to characterize the antibiotic resistance expressed by isolates. Particular resistance mechanisms may be suspected when the bacterium is resistant to several antibiotics in the same family (for example β-lactam agents). Methods: Using the disc agar diffusion test, a simple method was developed to distinguish between the common β-lactam resistance phenotypes of P. aeruginosa and, consequently, the possible resistance mechanism(s). Over a period of 5 years, we analysed 6300 P. aeruginosa strains isolated from various pathological specimens collected from different wards of Cochin Port-Royal Hospital, and reference and collection strains. Each strain had the wild-type phenotype or an acquired resistance phenotype. Eight anti-pseudomonal β-lactams (ticarcillin, cefotaxime or moxalactam, cefepime or cefpirome, imipenem, ceftazidime, aztreonam, cefsulodin and ticarcillin + clavulanic acid) were used as phenotypic markers. Results: The following markers were sufficient to distinguish between the wild-type phenotype and the various acquired resistance phenotypes: β-lactamase synthesis, reduced cell wall permeability and/or increased expression of efflux transporters (active efflux). Detection of resistance phenotypes allows ‘interpretive reading’ of antibiotic susceptibility tests. Conclusions: Clearly, improved interpretation of antibiotic susceptibility tests is important for a better appreciation of the effect of antimicrobial agents on bacteria such as P. aeruginosa.