Setting epidemiological cut-off values for Aeromonas salmonicida disc diffusion data capable of discriminating between strains on the basis of their possession of sul1 genes

This study evaluated the ability of the disc diffusion protocols and epidemiological cut-off values published by the Clinical and Laboratory Standards Institute (CLSI) to detect sul1 containing strains of Aeromonas salmonicida. Molecular analysis of sixteen strains demonstrated the presence of sul1 in seven of them. Two sets of disc diffusion data for these sixteen strains, produced in two studies preformed five years, apart, were used in the evaluation. Data sets produced using a disc containing both sulfamethoxazole and trimethoprim, as recommended by CLSI, were analysed using the recommended epidemiological cut-off value. When this cut-off value was applied to the data generated in the first study, fifteen of the sixteen strains were categorised as wild-type. When it was applied to the data generated in the second study, all sixteen strains were categorised as wild-type.When the strain susceptibilities were investigated using discs that contained 100 μg sulfamethoxazole only, strains that were sul1 negative manifested zones ≥ 18 mm in one study and ≥ 24 mm in the other. None of the sul1 containing strains manifested any zones of inhibition in either study. A provisional epidemiological cut-off value (≥ 9 mm) has been suggested for sulfamethoxazole disc data generated under the test conditions specified by CLSI. Evidence is presented that it is highly likely that application of this cut-off value to such data would lead to the categorisation of all sul1 negative strains as wild type and all sul1 containing strains as non-wild type.It is argued that, in order to maximise their ability to detect the presence of sul1 containing strains, standardised monitoring and surveillance programmes should adopt protocols that specify the use of discs containing only sulfamethoxazole.

► The gene sul1 was detected in seven of sixteen strains of Aeromonas salmonicida. ► The susceptibility of the strains was analysed using CLSI disc diffusion protocols. ► CLSI recommends using a disc containing both sulfamethoxazole and trimethoprim. ► The CLSI protocol lacked the sensitivity to detect sul1 containing strains. ► Use of discs containing sulfamethoxazole only could detect sul1 containing strains.