Antimicrobial susceptibility of Avibacterium paragallinarum isolates from outbreaks of infectious coryza in Dutch commercial poultry flocks, 2008-2017

For broth microdilution testing a new growth medium, recently developed for susceptibility testing of Haemophilus parasuis, was used. The medium proved to be suitable for broth microdilution susceptibility testing of NAD dependent Av. paragallinarum as well; visible growth was obtained in growth control wells and accepting a deviation of one dilution step, MIC values were reproducible. Results of 44 field isolates originating from 25 outbreaks showed relatively good susceptibility to antimicrobial agents that are recommended for the treatment of infectious coryza in the Netherlands, except for tetracycline; circa 75% of the isolates were characterized by MIC values of tetracycline of ≥16 μg/ml. In almost a quarter of these isolates with high MICs of tetracycline, tet genes were detected. For the remaining isolates with elevated MIC values, the mechanism conferring resistance remains to be studied. Of most agents, low MIC values were determined for the nine Kume and one Page serovar reference strains, as well as negative PCR results for resistance genes, being concordant with agar diffusion results reported for these strains.