Antibiotic susceptibility was tested by the disk diffusion method (Kirby Bauer). Antibiotic test disks were placed on agar plates with Mueller Hinton agar supplemented with 5% horse blood (MHF) inoculated with an A. butzleri bacterial suspension (MacFarland: 0.5). The test agar plates were incubated at 42°C under microaerophilic conditions for 24 h. The microaerophilic atmosphere was generated using a Whitley jar gassing system (Don Whitley Scientific Limited, Bingley, UK). Bacterial isolates with insufficient growth after 24 h were re-incubated, and the inhibition zone was measured after a total of 40–48 h. Since there are no breakpoints defined for Arcobacter neither according to the European Committee on Antimicrobial Susceptibility Testing (EUCAST) nor the Clinical & Laboratory Standards Institute (CLSI), the zone diameter was simply measured and an epidemiological cut-off (Ecoff) value determined on the basis of all tested isolates (for the definition of Ecoff values, see results). Isolates were tested for tetracycline (TE30), ciprofloxacin (CIP5), erythromycin (E15), gentamicin (CN10), and ampicillin (AMP10). Antimicrobial test discs were obtained from Oxoid/Thermo Fisher Scientific (Wesel, Germany). The laboratory work was carried out in a laboratory accredited according to DIN EN ISO 15189. The quality control strains used for resistance testing by agar diffusion were Campylobacter jejuni ATCC 33560 for the antibiotic test discs CIP5 (5 µg ciprofloxacin per disc), E15 (15 µg erythromycin per disc), and TE30 (30 µg tetracycline per disc), Pseudomonas aeruginosa ATCC 27853 for CN10 (10 µg gentamicin per disc), and Enterococcus faecalis ATCC 29212 for AMP10 (10 µg ampicillin per disc). Resistance to the three classes of antibiotics comprised of tetracyclines, fluoroquinolones, and macrolides was defined as multidrug resistance (MDR).
Free full text: Click here
