Epidemiological cut-off values for disc diffusion data generated by standard test protocols from Edwardsiella tarda and Vibrio harveyi

The susceptibility of 52 isolates of Edwardsiella tarda and 54 isolates of Vibrio harveyi to 12 antibiotics was established using internationally standardised disc diffusion protocols. Normalised resistance interpretation was employed to generate statistically valid epidemiological cut-off values for these susceptibility data. For three of these 24 species/antibiotic data sets, there were insufficient fully sensitive strains to allow analysis and for an additional two there were sufficient fully susceptible isolates to allow only a provisional estimate to be made. The data sets for a further two species/antibiotic combinations were considered too diverse to allow the calculation of a valid cut-off value. Valid epidemiological cut-off values were, therefore, calculated for 17 species/antibiotic combinations. It is argued that these cut-off values will greatly facilitate the performance of studies designed to monitor the consequence of antibiotic use in aquaculture. They will also facilitate the prudent and rational choice of antibiotics to be applied therapeutically in commercial farming of aquatic animals.