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JAC Advance Access originally published online on May 20, 2009
Journal of Antimicrobial Chemotherapy 2009 64(2):310-316; doi:10.1093/jac/dkp184
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© The Author 2009. 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

Original research

Comparison of three methods for susceptibility testing of Mycobacterium avium subsp. paratuberculosis to 11 antimicrobial drugs

Manju Y. Krishnan, Elizabeth J. B. Manning and Michael T. Collins*

Department of Pathobiological Sciences, School of Veterinary Medicine, University of Wisconsin–Madison, 2015 Linden Drive, Madison, WI 53706-110, USA

Received 28 January 2009; returned 3 March 2009; revised 24 March 2009; accepted 27 April 2009

* Corresponding author. Tel: +1-608-262-8457; Fax: +1-608-265-6463; E-mail: mcollin5{at}wisc.edu

Objectives: To evaluate the BACTECTM MGITTM 960/MGIT Para TB (MGIT) system for drug susceptibility testing of Mycobacterium avium subsp. paratuberculosis (MAP), a pathogen implicated in some forms of Crohn's disease.

Methods: MICs of 11 drugs for 10 MAP strains were determined using the MGIT system, the BACTECTM460TB system (BACTEC) and conventional agar dilution methods.

Results: MICs determined by MGIT methods showed 80%–100% agreement (±1 log2 dilution) with those determined by the BACTEC and agar dilution methods for ciprofloxacin, levofloxacin, azithromycin and clofazimine. The MGIT and BACTEC methods showed 70%, 80% and 90% agreement (±1 log2 dilution) for MICs of ethambutol, rifabutin and rifampicin; agreement for all drugs increased to 100% at 2 log2 dilution differences. For clarithromycin, the MGIT method had greater agreement with the agar dilution method (70% at the same dilution) than the BACTEC method (60% at ±1 log2 dilution); agreement increased to 100% at ±2 log2 dilutions in both cases. The MGIT and agar dilution methods agreed 60% and 100% for amikacin MICs at ±1 log2 dilution and ±2 log2 dilutions, respectively. By all methods MICs were higher than achievable serum concentrations for isoniazid and dapsone. There was 100% agreement between all three methods for azithromycin, clarithromycin and ciprofloxacin, and 80% agreement for rifampicin using published MIC thresholds available for M. avium complex strains.

Conclusions: This study shows that the MGIT system can be used for rapid and reliable drug susceptibility testing of MAP.

Keywords: agar dilution , BACTEC , MGIT , MIC


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M. Y. Krishnan, E. J. B. Manning, and M. T. Collins
Effects of interactions of antibacterial drugs with each other and with 6-mercaptopurine on in vitro growth of Mycobacterium avium subspecies paratuberculosis
J. Antimicrob. Chemother., November 1, 2009; 64(5): 1018 - 1023.
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