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JAC Advance Access published online on March 8, 2006
Journal of Antimicrobial Chemotherapy, doi:10.1093/jac/dkl054
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© The Author 2006. Published by Oxford University Press on behalf of the British Society for Antimicrobial Chemotherapy. All rights reserved. For Permissions, please e-mail: journals.permissions@oxfordjournals.org
Received September 5, 2005
Revised January 20, 2006
Accepted February 10, 2006

Brief report

Evaluation of the nitrate-based colorimetric method for testing the susceptibility of Mycobacterium tuberculosis to streptomycin and ethambutol in liquid cultures

H. Syre 1, H. Valvatne 1, P. Sandven 2, and H. M. S. Grewal 1 *

1 Section of Microbiology and Immunology, The Gade Institute, University of Bergen and Haukeland University Hospital, N-5021 Bergen, Norway
2 Division of Infectious Disease Control, Norwegian Institute of Public Health, PO Box 4404 Nydalen, 0403 Oslo, Norway

* To whom correspondence should be addressed.
H. M. S. Grewal, E-mail: Harleen.Grewal{at}cih.uib.no


  Abstract

Objectives: To evaluate the inexpensive colorimetric nitrate reductase-based antibiotic susceptibility (CONRAS) assay for testing the susceptibility of Mycobacterium tuberculosis to streptomycin and ethambutol in liquid cultures, and to compare the CONRAS test with the manual mycobacteria growth indicator tube (MGIT) test, using the radiometric BACTEC 460TB method as reference.

Methods: A total of 89 M. tuberculosis isolates were tested for susceptibility to streptomycin and ethambutol using the CONRAS and manual MGIT methods and the results were compared with BACTEC 460TB. Isolates with discrepant results between the CONRAS test and BACTEC 460TB were analysed using the agar proportion method, Etest and mutation analysis of genes involved in resistance to streptomycin and ethambutol.

Results: The agreement between the CONRAS test and BACTEC 460TB was 88% for streptomycin and 84% for ethambutol. The corresponding agreement of the manual MGIT test with BACTEC 460TB was 89 and 80%, respectively. There was good agreement for streptomycin and moderate agreement for ethambutol between the CONRAS and manual MGIT tests on one hand and BACTEC 460TB on the other (CONRAS test, kappastreptomycin 0.74 and kappaethambutol 0.59, P < 0.001; manual MGIT test, kappastreptomycin 0.77 and kappaethambutol 0.50, P < 0.001).

Conclusions: There is good agreement for the two non-radiometric liquid culture methods (CONRAS and manual MGIT) compared with BACTEC 460TB for the detection of streptomycin resistance. Further standardization is needed for testing of ethambutol resistance using the CONRAS and manual MGIT assays.

Keywords: antibiotic susceptibility tests; rapid methods; nitrate reductase.
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