The advantages of the use of discs containing single agents in disc diffusion testing of the susceptibility of Aeromonas salmonicida to potentiated sulphonamides

Abstract

The susceptibilities of 106 strains of Aeromonas salmonicida to trimethoprim/sulfamethoxazole (SFT) were determined in two laboratories using the Clinical and Laboratory Standards Institute's M42-A disc diffusion protocols. The data generated by the use of discs containing 25 μg SFT (SFT25) allowed the strains to be placed into two groups. Strains in one group (17 strains) generated no inhibition zones and the zones obtained from the other 89 strains were distributed over a wide range but showed no natural division into separate sub-classes. A further investigation performed by one of the participating laboratories, of the susceptibility of 91 of these 106 strains used discs containing 100 μg sulfmethoxazole (SFM100) and 5 μg trimethoprim (TMP5). Application of normalised resistance interpretation to these data allowed the estimation of epidemiological cut-off values for WT strains of ≥9 mm for SFM100 and ≥21 mm for TMP5. This investigation demonstrated the presence of three distinct phenotypic classes, one containing strains manifesting wild type susceptibility to both agents, another containing strains manifesting non-wild type susceptibility to both and a third containing strains manifesting wild type susceptibility with respect to TMP but non-wild type with respect to SFM. Analysis demonstrated the inability of SFT25 discs to generate data that allowed the separate identification of strains that were fully susceptible to both TMP and SFM from those that were fully susceptible to TMP but were not fully susceptible to SFM.It is recommended that, in investigation of the susceptibility to potentiated sulphonamides of isolates from diseased fish, separate discs, containing the individual components of the mixture, should be employed.