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Journal of Clinical Microbiology, June 2002, p. 1902-1907, Vol. 40, No. 6
0095-1137/02/$04.00+0     DOI: 10.1128/JCM.40.6.1902-1907.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.

Antimicrobial Susceptibility Testing of Bacillus anthracis: Comparison of Results Obtained by Using the National Committee for Clinical Laboratory Standards Broth Microdilution Reference and Etest Agar Gradient Diffusion Methods

M. Jasmine Mohammed,1 Chung K. Marston,2 Tanja Popovic,2 Robbin S. Weyant,2 and Fred C. Tenover1*

Epidemiology and Laboratory Branch, Division of Healthcare Quality Promotion,1 Meningitis and Special Pathogens Branch, Division of Bacterial and Mycotic Diseases, Centers for Disease Control and Prevention, Atlanta, Georgia 303332

Received 30 November 2001/ Returned for modification 7 January 2002/ Accepted 1 March 2002

We determined the patterns of antimicrobial susceptibility of 65 isolates of Bacillus anthracis (50 historical and 15 recent U.S. clinical isolates) to nine antimicrobial agents using the National Committee for Clinical Laboratory Standards (NCCLS) broth microdilution reference method. The results for the 50 historical B. anthracis isolates obtained by the broth microdilution method were compared to those generated by the Etest agar gradient diffusion method. One isolate of B. anthracis was ß-lactamase positive and resistant to penicillin (MIC, 128 µg/ml); a second isolate, which was ß-lactamase negative, was borderline penicillin resistant, with the penicillin MICs for the isolate varying from 0.12 to 0.25 µg/ml; and the remainder of the isolates were ß-lactamase negative and penicillin susceptible (MICs, <=0.12 µg/ml). Approximately 78% of the isolates showed reduced susceptibility to ceftriaxone (MICs, >=16 µg/ml). All B. anthracis isolates were susceptible to chloramphenicol (MICs, <=8 µg/ml), ciprofloxacin (MICs, <= 1 µg/ml), clindamycin (MICs, <=0.5 µg/ml), rifampin (MICs, <=0.5 µg/ml), tetracycline (MICs, <=0.06 µg/ml), and vancomycin (MICs, <=2 µg/ml) by use of NCCLS breakpoints for staphylococci. All 15 recent B. anthracis isolates from the United States were susceptible to penicillin, doxycycline, and ciprofloxacin. By use of the susceptibility breakpoint for staphylococci of <=0.5 µg/ml, 97% of the B. anthracis isolates tested would have been categorized as intermediate to erythromycin. No statistically significant difference was found between the results of broth microdilution testing and the results of the Etest method for any of the antimicrobial agents tested; however, the results for penicillin obtained by the Etest were 1 to 9 dilutions lower than those obtained by the broth microdilution method. The differences in the penicillin MICs by the Etest method and the difficulties of reading the Etest results through the glass of a biological safety cabinet may limit the utility of this alternate susceptibility testing method for B. anthracis isolates.

* Corresponding author. Mailing address: Division of Healthcare Quality Promotion (G08), Centers for Disease Control and Prevention, 1600 Clifton Rd., Atlanta, GA 30333. Phone: (404) 639-3375. Fax: (404) 639-1381. E-mail: fnt1{at}cdc.gov.

Journal of Clinical Microbiology, June 2002, p. 1902-1907, Vol. 40, No. 6
0095-1137/02/$04.00+0     DOI: 10.1128/JCM.40.6.1902-1907.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.



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Copyright © 2002 by the American Society for Microbiology. All rights reserved.