Inaccuracy of Susceptibility Testing 1 in Campylobacter Between the Disk Diffusion and 2 the Agar Dilution Methods

21 The agar dilution method has been standardized by the CLSI for the susceptibility testing of 22 Campylobacter species, and according to these standards, the disk diffusion method should only 23 be used for the screening of macrolide and ciprofloxacin resistance. Nevertheless, the disk 24 diffusion test is currently widely used, since it is easy to perform in clinical microbiology 25 laboratories. In this study, the disk diffusion method was compared to the agar dilution method 26 by analyzing the in vitro activities of seven antimicrobial agents against 174 Campylobacter 27 strains collected in Finland between 2003-2008. Recommendations of the CLSI were followed 28 using Muller-Hinton agar plates with 5% of sheep blood. For each strain, the disk diffusion tests 29 were performed two to four times. Of the 33 erythromycin-resistant strains (MIC ≥ 16 μg/ml), 24 30 (73%) constantly showed a 6-mm (no inhibition) erythromycin inhibition zone, while for seven 31 strains the inhibition zone varied from 6–44 mm in repeated measurements. Among the 141 32 erythromycin-susceptible strains (MIC < 16 μg/ml), erythromycin inhibition zones varied 33 between 6–61 mm. Of the 87 ciprofloxacin-resistant strains, 47 (54%) showed 6-mm inhibition 34 zones, while 40 strains showed inhibition zones between 6-60 mm. Significant differences between 35 the repetitions were observed in the disk diffusion for all antimicrobial agents and all strains 36 except for the macrolide-resistant strains regarding the macrolides. For 17 (10%) strains, the 37 variation in repeated measurements was substantial. These results show that the disk diffusion 38 method may not be a reliable tool for the susceptibility testing of Campylobacter spp. Further 39 studies are needed to assess whether the disk diffusion test could be improved or whether all 40 susceptibilities of campylobacters should be tested using an MIC-based method. 41 42 on O cber 1, 2017 by gest ht://jcm .sm .rg/ D ow nladed fom