Development of a harmonized method for antimicrobial susceptibility testing of Bordetella avium using broth microdilution and detection of resistance genes

Aims: In response to a request from the Clinical and Laboratory Standards Institute (CLSI), the objective of this study was to develop a harmonized method for broth microdilution susceptibility testing of Bordetella (B.) avium, the major causative agent of infectious coryza in poultry.

Methods and results: To find a suitable test medium, growth curves with four epidemiologically unrelated B. avium isolates were created in cation-adjusted Mueller-Hinton broth (CAMHB), CAMHB + 2.5% lysed horse blood and veterinary fastidious medium. All isolates showed good growth in CAMHB, therefore MIC values were determined using this medium and the homogeneity of the values was determined. An essential MIC agreement of 99.7% was calculated. Testing of a larger strain collection (n = 49) for their susceptibility to 24 antimicrobials confirmed the suitability of the tested method and revealed some isolates with elevated MICs of florfenicol (n = 1), streptomycin (n = 2), tetracyclines (n = 5), and trimethoprim/sulfamethoxazole (n = 6). PCR assays detected the resistance genes aadA1, dfrB1, floR, sul1, sul2 and tet(A).

Conclusions: The method used enables easy reading and a good reproducibility of MIC values for B. avium.

Significance and impact of study: Application of the tested method allows harmonized resistance testing of B. avium and identification of isolates with elevated MIC values.