Antimicrobial susceptibility and tetracycline resistance determinant genotyping of Gallibacterium anatis

The present investigation was undertaken to assess the antimicrobial susceptibility of a collection of 58 Gallibacterium isolates. All strains were tested by the broth dilution method using the veterinary fastidious medium. A total of 46 field strains were tested, whereof 23 were clinical isolates from 10 Mexican layer flocks and another 23 isolates originated from 13 clinically healthy poultry flocks in Denmark. In addition, 12 Gallibacterium reference strains that had been isolated some 30-40 years ago were included. The 58 strains were tested against 23 compounds of different classes. Multidrug resistance (resistance towards≥three drugs) was observed for 65% of the field strains and only two strains were susceptible to all compounds. Most prominently, resistance to tetracycline and sulfamethoxazole was observed in 92% and 97% of the field strains, respectively. For comparison these figures were 67% and 42%, respectively, for the reference strains. Genotyping of tetracycline resistance determinants was performed with primers specific for tet(A-E, H, K-M, O). Strains positive for tet(B), tet(H) and tet(L) were identified, however, in 20 out of 49 tetracycline resistant strains no determinant was identified. This is the first study to determine the antimicrobial susceptibility of Gallibacterium anatis by MIC revealing that multidrug resistance is very common among G. anatis field isolates. tet(B) was by far the most common determinant identified but future work should aim at identifying the tetracycline resistance determinants in the remaining 41% of strains where no determinant was assigned.