THE INHIBITION BY IONOPHORES IN-VITRO OF AN ENTEROCOCCUS-LIKE PATHOGEN OF RAINBOW-TROUT, ONCORHYNCHUS-MYKISS

Streptococcosis is a major disease of several fish species in Australia, Japan and South Africa. The minimum inhibitory concentration of some ionophores (lasalocid, monensin, narasin and salinomycin) was determined in vitro for an Enterococcus-like species pathogenic for rainbow trout (Oncorhynchus mykiss) in Australia. Forty isolates of the fish pathogen were tested, together with control strains of Staphylococcus aureus ATCC 29213, Enterococcus faecalis ATCC 29212 and Streptococcus bovis ATCC 9809. The minimum inhibitory concentrations (MIC) of erythromycin, the drug of choice for controlling streptococcosis, ranged between 0.1 and 0.8 mu g/ml whereas the MIC values for the ionophores ranged between 0.2 and 1.5 mu g/ml. Of the ionophores tested, narasin was the most inhibitory (0.2-0.4 mu g/ml), while monensin was the least inhibitory (0.4-1.5 mu g/ml). Salinomycin was marginally more inhibitory (0.4-0.8 mu g/ml) than lasalocid (0.8 mu g/ml).