Application of Normalised Resistance Interpretation to the detection of multiple low-level resistance in strains of Vibrio anguillarum obtained from Greek fish farms

Disc diffusion methods were employed to characterise the distributions of susceptibility to oxolinic acid, oxytetracycline, trimethoprim/sulfamethoxazole and furazolidone in 38 strains of Vibrio anguillarum isolated from Greek fish farms. For each of the four agents one strain gave no zone of inhibition and these four strains could be identified, unambiguously, as manifesting non-wild type (NWT) susceptibility. Visual examination of the distributions of zone sizes obtained with oxolinic acid discs for the remaining strains suggested that 4-5 of these had a reduced susceptibility and could also be classified as NWT. The zone sizes obtained for the other three agents were distributed within a single group that, in all cases, showed a significant skew towards lower values. Normalised Resistance Interpretation (NRI) was used to investigate these data and this analysis suggested that for each of the three agents, oxytetracycline, trimethoprim/sulfamethoxazole and furazolidone, there were grounds for classifying further seven strains as non-wild type and, therefore, as manifesting a low-level resistance phenotype. In total, 11 strains were identified as manifesting low-level resistance to at least one of the four agents. Of these 11 strains, 2 were classified as manifesting a low-level resistance to all four agents and 3 showed this phenotype with respect to at least three of the agents. Although this analysis suggests that multiple low-level resistance might be reasonably common in V. anguillarum there are, at present, no data that allow any prediction to be made as to the probable clinical significance of such strains.