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Journal of Clinical Microbiology, January 2002, p. 26-30, Vol. 40, No. 1
0095-1137/02/$04.00+0     DOI: 10.1128/JCM.40.1.26-30.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.

Comparison of Two Culture Methods for Detection of Tobramycin-Resistant Gram-Negative Organisms in the Sputum of Patients with Cystic Fibrosis

Jill M. Van Dalfsen,1 Jenny R. Stapp,2 Charles Phelps,1,{dagger} Patricia Stewart,1,{ddagger} and Jane L. Burns2*

Chiron Corporation, Department of Pediatrics, University of Washington, Children’s Hospital and Regional Medical Center, Seattle, Washington,1 Division of Infectious Disease, Department of Pediatrics, University of Washington, Children’s Hospital and Regional Medical Center, Seattle, Washington2

Received 16 July 2001/ Returned for modification 29 August 2001/ Accepted 15 October 2001

A culture method utilizing quantitative plating on antibiotic-containing media has been proposed as a technique for the detection of tobramycin-resistant organisms that is more sensitive than standard methods. Typical sputum culture methods quantitate the relative amounts of each distinct morphotype, followed by antibiotic susceptibility testing of a single colony of each morphotype. Sputum specimens from 240 cystic fibrosis patients were homogenized, serially diluted, and processed in parallel by the standard method (MacConkey agar and OF basal medium with agar, polymyxin, bacitracin, and lactose) and by plating on antibiotic-containing media (MacConkey agar with tobramycin added at 25 µg/ml [MAC-25] and 100 µg/ml [MAC-100]). MICs of tobramycin were determined for all Pseudomonas aeruginosa isolates by broth microdilution. Growth of P. aeruginosa on MAC-25 was considered to be equivalent to a tobramycin MIC of >=16 µg/ml, and growth on MAC-100 was considered to be equivalent to a tobramycin MIC of >=128 µg/ml. Analysis of method-specific detection rates showed that tobramycin-containing medium was more sensitive than the standard method for the detection of tobramycin-resistant P. aeruginosa, Stenotrophomonas maltophilia, and Achromobacter xylosoxidans but was less sensitive for the detection of Burkholderia cepacia than the standard method. When MICs for P. aeruginosa that grew on tobramycin-containing medium were tested by broth microdilution, the MICs for 28 of 121 strains (23%) growing on MAC-25 and 22 of 56 strains (39%) growing on MAC-100 were MICs <16 and <128 µg/ml, respectively. Addition of a tobramycin-containing MacConkey plate to the routine media for sputum culture may provide additional, clinically relevant microbiologic data.

* Corresponding author. Mailing address: Division of Infectious Disease, Children’s Hospital and Regional Medical Center, 4800 Sand Point Way N.E., CH-32, Seattle, WA 98105. Phone: (206) 526-2073. Fax: (206) 527-3890. E-mail: jburns{at}chmc.org.

{dagger} Present address: ICOS Corporation, Seattle, Wash.

{ddagger} Present address: Corixa Corporation, Seattle, Wash.

Journal of Clinical Microbiology, January 2002, p. 26-30, Vol. 40, No. 1
0095-1137/02/$04.00+0     DOI: 10.1128/JCM.40.1.26-30.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.



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