In Vitro and in Vivo Susceptibility of Streptococcus iniae, Isolated from Diseased Fish, to Antimicrobial Agents

Abstract

In vitro activity of 36 antimicrobial agents were tested against 15 strains of Streptococcus iniae isolated from diseased cultured yellowtail, Seriola quinqueradiata, Japanese flounder, Paralichthys olivaceus, ayu, Plecoglossus altivelis, amago salmon, Oncorhynchus rhodurus, and natural common mackerel, Scomber japonicas by standard agar plate dilution method. In vivo therapeutic effects of selected 12 drugs, to which S. iniae is highly susceptible, were also observed in experimentally infected yellowtail. Penicillins, cephalosporines, macrolides, tetracyclines, rifampicin, lincomycin, sodium nifurstyrenate, and diaminopyrimidines were highly effective (MIC, 0.00313-0.78μg/ml) against all the strains tested. Chloramphenicols and bacitracin were also effective (MIC, 0.78-3.13μg/ml) . Susceptibility of S. iniae to penicillins, cephalosporines, rifampicin, bacitracin, sodium nifurstyrenate, and diaminopyrimidines was higher than that of Enterococcus seriolicida. From the results of therapeutic activity of selected drugs such as penicillins, macrolides, tetracyclines, rifampicin, lincomycin, sodium nifurstyrenate, and diaminopyrimidines to diseased yellowtail, it can be considered that these drugs are effective against streptococosis caused by S. iniae.