Comparison of agar dilution and antibiotic gradient strip test with broth microdilution for susceptibility testing of swine Brachyspira species

Connie J. Mirajkar, Connie J Gebhart

Research output: Contribution to journalArticlepeer-review

9 Scopus citations


Production-limiting diseases in swine caused by Brachyspira are characterized by mucohemorrhagic diarrhea (B. hyodysenteriae and “B. hampsonii”) or mild colitis (B. pilosicoli), while B. murdochii is often isolated from healthy pigs. Emergence of novel pathogenic Brachyspira species and strains with reduced susceptibility to commonly used antimicrobials has reinforced the need for standardized susceptibility testing. Two methods are currently used for Brachyspira susceptibility testing: agar dilution (AD) and broth microdilution (BMD). However, these tests have primarily been used for B. hyodysenteriae and rarely for B. pilosicoli. Information on the use of commercial susceptibility testing products such as antibiotic gradient strips is lacking. Our main objective was to validate and compare the susceptibility results, measured as the minimum inhibitory concentration (MIC), of 6 antimicrobials for 4 Brachyspira species (B. hyodysenteriae, “B. hampsonii”, B. pilosicoli, and B. murdochii) by BMD and AD (tiamulin, valnemulin, lincomycin, tylosin, and carbadox) or antibiotic gradient strip (doxycycline) methods. In general, the results of a high percentage of all 4 Brachyspira species differed by ±1 log2 dilution or less by BMD and AD for tiamulin, valnemulin, lincomycin, and tylosin, and by BMD and antibiotic gradient strip for doxycycline. The carbadox MICs obtained by BMD were 1–5 doubling dilutions different than those obtained by AD. BMD for Brachyspira was quicker to perform with less ambiguous interpretation of results when compared with AD and antibiotic gradient strip methods, and the results confirm the utility of BMD in routine diagnostics.

Original languageEnglish (US)
Pages (from-to)133-143
Number of pages11
JournalJournal of Veterinary Diagnostic Investigation
Issue number2
StatePublished - Mar 1 2016

Bibliographical note

Funding Information:
The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was partially supported by Novartis Pharmaceuticals Corp. [11651-GEBHART-NOVARTIS PHARM-CON000000033714-3000].

Publisher Copyright:
© 2016, © 2016 The Author(s).

Copyright 2016 Elsevier B.V., All rights reserved.


  • Agar
  • Brachyspira
  • antibiotic gradient strip
  • antimicrobial
  • broth
  • minimum inhibitory concentration
  • susceptibility


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