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Journal of Clinical Microbiology, September 1999, p. 2987-2991, Vol. 37, No. 9
Department of Pediatrics and Clinical
Microbiology, Columbia University, New York, New York
100321; Department of Pediatrics,
Children's Hospital and Regional Medical Center and University of
Washington, Seattle, Washington 981052; and
Department of Medicine and Clinical Microbiology, University of
Iowa, Iowa City, Iowa 522423
Received 23 November 1998/Returned for modification 26 February
1999/Accepted 13 April 1999
The development of multidrug-resistant Pseudomonas
aeruginosa in patients with cystic fibrosis (CF) is most likely a
consequence of increasing life expectancy and more prolonged exposure
to antibiotics. The optimal method for antibiotic susceptibility
testing of CF strains, particularly mucoid P. aeruginosa
strains, is unknown. Antimicrobial susceptibilities of 48 CF strains
(25 mucoid) and 50 non-CF strains to 12 anti-Pseudomonas
agents were tested by both agar dilution and commercially
custom-prepared broth microdilution plates (PML Microbiologicals,
Portland, Oreg.) in three laboratories simultaneously to determine if
broth microdilution could substitute for agar dilution as the reference
method in subsequent studies. Comparison of MICs generated by agar
dilution and broth microdilution demonstrated correlation coefficients
(r) exceeding 0.85 for all agents tested; correlation was
excellent for aminoglycosides (r
0095-1137/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.
Evaluation of Reference Dilution Test Methods for
Antimicrobial Susceptibility Testing of Pseudomonas
aeruginosa Strains Isolated from Patients with Cystic
Fibrosis
0.92) and very
good for
-lactam agents including agents paired with a
-lactamase
inhibitor (r
0.87) and for ciprofloxacin (r = 0.86). Correlation was not improved by 48-h
readings, but correlation between 24- and 48-h readings ranged between
0.91 and 0.98 for both methods. Interlaboratory variations were
minimal, as the percentage of acceptable variations was 94% for both
methods, and serious discords were infrequent (<2% of comparisons).
However, CF strains were more likely to have serious discords than were non-CF strains (P < 0.0001), although mucoid strains
were not more likely to have serious discords than were nonmucoid
strains. In this study, MICs determined by custom-prepared broth
microdilution compared favorably with MICs determined by agar dilution.
Thus, this broth microdilution assay can serve as a reference method and facilitate future studies to determine the optimal method for
antibiotic susceptibility testing of CF strains.
*
Corresponding author. Mailing address: Columbia
University, 650 West 168th St., PH4W-470 New York, NY 10032. Phone:
(212) 305-9446. Fax: (212) 305-9491. E-mail:
LS5{at}columbia.edu.
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