Setting an epidemiological cut-off value for the single plate protocol from disc diffusion data generated for Aeromonas salmonicida

The single plate protocol was applied to generate susceptibility measures for 50 strains of Aeromonas salmonicida against five antibiotic agents. Analysis of the resultant data demonstrated that a single epidemiological cut-off value could be set that was appropriate for all 5 agents. The classification of strains as wild type or non-wild type by the application of this cut-off value showed complete agreement with the classification produced by applying normalised resistance interpretation generated cut-off values to the data generated by the standard Clinical and Laboratory Standards Institute protocols.Consideration of the design features of the single plate protocol and the published disc diffusion data for bacteria isolated from aquatic animals suggests that the application of this single cut-off value is valid for many, if not all, agents and species and in all laboratories. It is argued that the existence of a single universal cut-off value adds to the already considerable advantages that the single plate protocol presents for laboratories involved in clinical diagnosis of disease epizootics in aquaculture.

► The susceptibility of A. salmonicida was measured using the single plate protocol.
► A single epidemiological cut-off value (ECV) could be applied to data for 5 agents.
► The categorisation of strains showed 100% agreement with standard approaches.
► The possibility that the application of the ECV is universally valid is discussed.