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Susceptibility

Detection and Susceptibility Testing of Hypermutable Pseudomonas aeruginosa Strains with the Etest and Disk Diffusion

Maria D. Maciá, Nuria Borrell, José L. Pérez, Antonio Oliver
Maria D. Maciá
Servicio de Microbiología, Hospital Son Dureta, Palma de Mallorca, Spain
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Nuria Borrell
Servicio de Microbiología, Hospital Son Dureta, Palma de Mallorca, Spain
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José L. Pérez
Servicio de Microbiología, Hospital Son Dureta, Palma de Mallorca, Spain
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Antonio Oliver
Servicio de Microbiología, Hospital Son Dureta, Palma de Mallorca, Spain
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  • For correspondence: aoliver@hsd.es
DOI: 10.1128/AAC.48.7.2665-2672.2004
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  • FIG. 1.
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    FIG. 1.

    Etest antibiograms of PAO1 and PAOΔmutS with ceftazidime, imipenem, meropenem, ciprofloxacin, and tobramycin.

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    FIG. 2.

    Disk diffusion antibiograms of PAOΔmutS and PAO1 with ceftazidime (CAZ), imipenem (IPM), meropenem (MEM), ciprofloxacin (CIP), and tobramycin (TOB).

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    FIG. 3.

    Distributions of MICs of ceftazidime, imipenem, meropenem, ciprofloxacin, and tobramycin. White bars, percentages of nonhypermutable isolates; black bars, percentages of hypermutable isolates considering resistant mutant subpopulations; gray bars, percentages of hypermutable isolates without considering mutant subpopulations.

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  • TABLE 1.

    MICs for and inhibition zone diameters of strains PAO1 and PAOΔmutS and their respective resistant mutant subpopulations

    AntibioticPAO1aPAOΔmutSaNCCLS breakpointsc
    Etest MIC (μg/ml)Disk diffusion diam (mm)EtestDisk diffusionN, R MIC (μg/ml)R, N diam (mm)
    MIC (μg/ml)MIC for the subpopulation (relative no.)bDiam (mm)Diam of the subpopulation (relative no.)
    Ceftazidime125.6148 (+++)25.414.1 (+++)≤8, ≥32≤14, ≥18
    Imipenem1.521.91.512 (+ to ++)22.814.8 (++)≤4, ≥16≤13, ≥16
    Meropenem0.3829.40.53 (++)27.219.0 (++)≤4, ≥16≤13, ≥16
    Ciprofloxacin0.06432.40.0640.38 (++)34.029.6 (++)≤1, ≥4≤15, ≥12
    Tobramycin1.520.023 (+)18.015.0 (+)≤4, ≥16≤12, ≥15
    • ↵ a Results shown are the median values of five independent determinations. MICs and inhibition zone diameters were measured at 24 h for the general population and at 36 h for the resistant mutant subpopulation. The highest MICs or the smallest inhibition zone diameters of resistant mutant subpopulations are listed.

    • ↵ b Relative number of resistant mutant colonies (see Materials and Methods). +, < 10 mutants; ++ 10 to 100 mutants; +++, > 100 mutants.

    • ↵ c N, nonsuceptibility breakpoint; R, resistance breakpoint.

  • TABLE 2.

    Numbers of nonhypermutable or hypermutable isolates for which resistant mutant subpopulations were observed for each antibiotic

    AntibioticNo. of isolates with resistant mutants subpopulations observed/no. tested bya:
    EtestDisk diffusion
    Nonhypermutable isolates (n = 25)Hypermutable isolates (n = 10)Nonhypermutable isolates (n = 25)Hypermutable isolates (n = 10)
    Ceftazidime3/259/92/259/9
    Imipenem2/258/84/259/9
    Meropenem3/2510/105/2510/10
    Ciprofloxacin2/259/101/258/10
    Tobramycin1/246/101/247/10
    • ↵ a Isolates for which the MIC was higher than the highest concentration on the Etest strips were excluded (see Materials and Methods).

  • TABLE 3.

    Numbers of antibiotics for which mutants were detected in nonhypermutable or hypermutable isolates

    Type of isolates (n)TestNo. of antibiotics for which mutants were detected/no. tested
    ≥1≥2≥3≥45
    Hypermutable (10)Etest10/1010/1010/109/103/10
    Disk diffusion10/1010/1010/109/102/5
    Nonhypermutable (25)Etest9/253/250/250/250/25
    Disk diffusion9/255/250/250/250/25
  • TABLE 4.

    Antibiotic susceptibility percentages and mean MICs and inhibition zones diameters of nonhypermutable isolates, hypermutable isolates, and hypermutable isolates considering resistant mutant subpopulations

    AntibioticNonhypermutable isolates (n = 25)aHypermutable isolates (n = 10)aHypermutable isolates + subpopulations (n = 10)a,bNCCLS breakpointsc
    % S by Etest/ddMean Etest/MIC (μg/ml)Mean dd diam (mm)% S by Etest/ddMean Etest/MIC (μg/ml)Mean dd diam (mm)% S by Etest/ddMean Etest/MIC (μg/ml)Mean diam, dd (mm)N, R MIC (μg/ml)R, N diam (mm)
    Ceftazidime96/961.328.460/708.419.910/10129.77.6≤8, ≥32≤14, ≥18
    Imipenem100/961.122.150/606.315.610/1028.29.1≤4, ≥16≤13, ≥16
    Meropenem100/1000.136.5100/1000.429.760/801.720.8≤4, ≥16≤13, ≥16
    Ciprofloxacin92/920.327.420/201.917.520/207.911.6≤1, ≥4≤15, ≥21
    Tobramycin84/722.415.990/703.416.770/504.512.6≤4, ≥16≤12, ≥15
    • ↵ a S, percentage of strains susceptible according to NCCLS breakpoints by the Etest and by disk diffusion (dd). Means are geometric means.

    • ↵ b Resistant mutant subpopulations were considered for these values.

    • ↵ c N, nonsusceptibility breakpoint; R, resistance breakpoint.

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Detection and Susceptibility Testing of Hypermutable Pseudomonas aeruginosa Strains with the Etest and Disk Diffusion
Maria D. Maciá, Nuria Borrell, José L. Pérez, Antonio Oliver
Antimicrobial Agents and Chemotherapy Jun 2004, 48 (7) 2665-2672; DOI: 10.1128/AAC.48.7.2665-2672.2004

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Detection and Susceptibility Testing of Hypermutable Pseudomonas aeruginosa Strains with the Etest and Disk Diffusion
Maria D. Maciá, Nuria Borrell, José L. Pérez, Antonio Oliver
Antimicrobial Agents and Chemotherapy Jun 2004, 48 (7) 2665-2672; DOI: 10.1128/AAC.48.7.2665-2672.2004
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KEYWORDS

mutation
Pseudomonas aeruginosa

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