Antimicrobial susceptibility testing of Arcobacter butzleri and Arcobacter cryaerophilus strains isolated from Belgian patients.

Objectives: To evaluate the feasibility of different methods for susceptibility testing of human Arcobacter isolates, to assess susceptibility to antibiotics commonly used to treat diarrhoeal illness and to obtain MIC distribution data.

Methods: One-hundred-and-six unique Arcobacter strains were collected during an epidemiological study on pathogens in gastroenteritis. Strains were identified by multiplex PCR and PCR-RFLP, and characterized by PFGE. Susceptibility to ampicillin, erythromycin, tetracycline, doxycycline, gentamicin and ciprofloxacin was determined using gradient strip and disc diffusion methodology. Optimal conditions for growth and incubation were tested. Azithromycin was tested with gradient strip diffusion on a subset of 40 strains. Sequence analysis of the quinolone resistance-determining region of gyrA was performed for a subset of 18 strains.

Results: Based on gradient diffusion results, most Arcobacter strains were susceptible to gentamicin (99%) and tetracycline (89%). Erythromycin (78%), ciprofloxacin (72%) and doxycycline (76%) retained moderate activity against Arcobacter spp. Only 9% of the strains were susceptible to ampicillin. Most Arcobacter butzleri strains were susceptible to ciprofloxacin (87%), whereas half of the Arcobacter cryaerophilus isolates (51%) showed high-level resistance (MIC >32 mg/L). MIC50 values were comparable for both macrolide antibiotics. Ciprofloxacin-resistant strains possessed an identical mutation in gyrA. Overall, categorical agreement between gradient and disc diffusion results was 60%. Gradient diffusion showed superior readability.

Conclusions: Gradient diffusion methodology is preferred for routine susceptibility testing. Acquired resistance to fluoroquinolones was observed in A. cryaerophilus. Macrolides are not first-choice empirical antibiotics for Arcobacter infections. Tetracyclines can be suggested for treatment of documented Arcobacter-related gastrointestinal infections.