title: Characterization of clinical isolates of Pseudomonas aeruginosa heterogeneously resistant to carbapenems.
abstract: Fourteen apparently carbapenem-susceptible Pseudomonas aeruginosa clinical isolates that exhibited colonies within the inhibition zone around carbapenem discs were analysed. MICs of carbapenems were determined and the isolates were genotyped by PFGE. Population analysis, one-step selection of carbapenem-resistant mutants and growth curves of progenitors and carbapenem-resistant subpopulations were performed. Agar dilution MICs of imipenem and meropenem ranged from 0.5 to 4 mg l(-1) and from 0.25 to 2 mg l(-1), respectively. Population analysis confirmed subpopulations that grew in concentrations of up to 18 mg l(-1) and 12 mg l(-1) of imipenem and meropenem, respectively, at frequencies ranging from 6.9 x 10(-5) to 1.1 x 10(-7), suggesting that they might not be detected by standard agar dilution MIC testing. The minority subpopulations exhibited MICs for imipenem ranging from 10 to 20 mg l(-1) and for meropenem from 4 to 14 mg l(-1). The one-step 8 mg l(-1) selection of imipenem-resistant mutants test showed growth in all isolates at frequencies ranging from 3.8 x 10(-4) to 5.1 x 10(-7). Growth curves revealed a prolonged lag phase and a short exponential phase for the heterogeneous subpopulations compared with their respective native subpopulations. These findings may be indicative that the use of carbapenems can lead to selection of P. aeruginosa resistant subpopulations that subsequently cause infections and result in treatment failure.
authors.affiliation: Department of Medical Microbiology, University of Thessalia, Mezourlo, Larissa, Greece.
authors.forename: Spyros
authors.initials: S
authors.lastname: Pournaras
authors.affiliation: Department of Medical Microbiology, University of Thessalia, Mezourlo, Larissa, Greece.
authors.forename: Alexandros
authors.initials: A
authors.lastname: Ikonomidis
authors.affiliation: Department of Microbiology, Medical School, University of Athens, Athens, Greece.
authors.forename: Antonios
authors.initials: A
authors.lastname: Markogiannakis
authors.affiliation: Department of Microbiology, Medical School, University of Athens, Athens, Greece.
authors.forename: Nicholas
authors.initials: N
authors.lastname: Spanakis
authors.affiliation: Department of Medical Microbiology, University of Thessalia, Mezourlo, Larissa, Greece.
authors.forename: Antonios N
authors.initials: AN
authors.lastname: Maniatis
authors.affiliation: Department of Microbiology, Medical School, University of Athens, Athens, Greece.
authors.forename: Athanassios
authors.initials: A
authors.lastname: Tsakris
mesh_terms: D000900:Anti-Bacterial Agents / Q000494:pharmacology; D015780:Carbapenems / Q000494:pharmacology*; D004305:Dose-Response Relationship, Drug; D024901:Drug Resistance, Multiple, Bacterial / Q000235:genetics*; D018740:Genetic Heterogeneity; D005838:Genotype; D006801:Humans; D015378:Imipenem / Q000494:pharmacology; D000077731:Meropenem; D008826:Microbial Sensitivity Tests; D050296:Microbial Viability / Q000187:drug effects / Q000235:genetics; D009154:Mutation; D011552:Pseudomonas Infections / Q000382:microbiology; D011550:Pseudomonas aeruginosa / Q000187:drug effects* / Q000235:genetics / Q000302:isolation & purification; D013845:Thienamycins / Q000494:pharmacology; D013997:Time Factors
chemical_list: D000900:Anti-Bacterial Agents; D015780:Carbapenems; D013845:Thienamycins; D015378:Imipenem; D000077731:Meropenem