Antimicrobial susceptibility testing of Arcobacter butzleri: development and application of a new protocol for broth microdilution

Objectives: To develop a standard reference broth microdilution method for antimicrobial susceptibility testing (AST) of Arcobacter butzleri. The protocol was subsequently applied to a collection of A. butzleri isolates from different sources. Methods: Broth microdilution susceptibility testing was performed on eight A. butzleri isolates in three media: non-supplemented CAMHB, CAMHB+2% FBS and CAMHB+5% FBS. The MIC values were read after 24 and 48 h of incubation at 35 +/- 2 degrees C in ambient air. A logistic regression model was used to determine the combination of medium and incubation time yielding the most homogeneous results. Subsequently, the protocol was applied to 65 A. butzleri isolates to determine their MICs of 31 antimicrobial agents. Results: The statistical analysis revealed that the most homogeneous MIC values were obtained with CAMHB+5% FBS and reading of MIC values after 24 h of incubation. The standardized method was successful for AST of all 65 A. butzleri isolates. MIC values were distributed unimodally for most antimicrobial agents. However, one field isolate showed elevated MIC values of gentamicin, streptomycin, tetracycline and trimethoprim/sulfamethoxazole. Conclusions: This study presents a new protocol for AST of A. butzleri by broth microdilution and shows the distribution of MIC values of 31 antimicrobial agents for a collection of A. butzleri isolates from different origins.