Data regarding antimicrobial susceptibilities of
Arcobacter spp. are scarce, mainly due to missing standardized protocols and defined breakpoints, which makes it difficult to interpret results and to define antimicrobial resistance. In previous studies, MIC results have been compared with breakpoints for
Enterobacteriaceae or
Staphylococcus spp. as defined by the Clinical Laboratory Standards Institute (CLSI), with breakpoints for
Campylobacter as defined by the U.S. National Resistance Monitoring System criteria or with EUCAST breakpoints for
Enterobacteriaceae, Campylobacter or non-species related breakpoints [
5,
30,
31]. In our study, we compared the MICs with ECOFFs defined by EUCAST for
C. jejuni [
32]. For ciprofloxacin, gentamicin and ampicillin the
C. jejuni ECOFFs appear to apply for
Arcobacter as well as previously proposed by Riesenberg et al. [
33]. However, our data suggest that
Arcobacter ECOFFs for erythromycin, tetracycline and azithromycin may be higher than those of
C. jejuni. All of our isolates displayed MICs for azithromycin above the ECOFF of
C. jejuni (0.25 µg/ml), which, however, is comparable with data from a Belgian study [
34]. Although erythromycin and azithromycin are both macrolides, the bimodal distribution for azithromycin but not for erythromycin was remarkable. Van den Abeele et al. have also detected MICs > 8 µg/ml for azithromycin in 50% of
A. butzleri isolates, which is in line with our results [
34]. Likewise, other studies revealed elevated MICs for azithromycin in up to 95% of
A. butzleri and in 20% of
A. cryaerophilus strains isolated from poultry products [
30,
35]. Similar to our results, other studies on antimicrobial susceptibility revealed also low MICs for
Arcobacter spp. to erythromycin whereas some studies reported resistance rates up to 62% [
5,
36,
37]. In contrast to our study, those studies used disc diffusion assays with 15 µg/disc and applied resistance criteria for
Enterobacteriaceae according to CLSI 2010. In
Campylobacter, there is usually cross-resistance between azithromycin and erythromycin. Single isolates, however, may display susceptibility to erythromycin and resistance to azithromycin, and whole genome sequencing analysis revealed an amino acid substitution in ribosomal protein L22 (leading to azithromycin resistance), but no mutations in the 23S rRNA gene, which explains the susceptibility to erythromycin [
38]. Further analyses are needed to determine the genomic background being responsible for the divergent MIC distributions observed by us for
Arcobacter spp.
As mentioned before, 86% of the investigated
Arcobacter isolates showed low MICs for ciprofloxacin ranging from 0.032–0.50 µg/ml, which is further supported by a recent study reporting ciprofloxacin susceptibility for all tested
Arcobacter butzleri isolates [
36]. In contrast, clinical
Campylobacter isolates displayed high resistance rates (MICs ≥ 4 µg/ml) ranging from 45 to 71.4% [
39,
40]. Notably, we found elevated MICs for ciprofloxacin predominantly in
A. cryaerophilus strains similar to a Belgian study [
34]. Thus, ciprofloxacin might be the drug of choice, if antibiotic treatment of
A. butzleri-infection is required.
In accordance with our data, only low resistance rates from 0–4% of
Arcobacter spp. to gentamicin have been reported before [
36]. Similarly, susceptibility to tetracycline might be common, although one recent study from retail food in Portugal demonstrated high resistance (95%) in
A. butzleri [
5,
41]. Furthermore, 42% of our
A. butzleri isolates displayed high MICs for ampicillin (24–64 µg/ml), which is similar to previous studies where 50 to 100% isolates with high ampicillin MICs have been shown [
20,
22,
31,
34].