Studies on Tobicillin, a New Antibiotic Drug for Enterococcicosis in Yellowtail Seriola quinqueradiata

Tobicillin (TBPC), an ester derivative of penicillin G (PCG), was examined as a treatment for enterococcicosis in yellowtail. After 6 h at 25 degree C at pH 3, the residual rate of TBPC was over 80%, but that of PCG was only 20%. TBPC was stable in an acidic solution (pH 3). The area under the blood concentration-time curve (AUC) between 0 and 10 h after oral administration was about 7 times higher for TBPC (35.2 mu g/ml times h) than that for PCG (5,2 mu g/ml times h). In drug sensitivity tests using 156 strains of field-isolated Enterococcus seriolicida, the MICs of PCG were 0.39-0.78 mu g/ml, and no strains were resistant to PCG. On the other hand, the MICs of erythromycin, lincomycin and oxytetracycline showed biphasic distributions that included both resistant and sensitive strains. In a preliminary evaluation of the efficacy of TBPC against experimental enterococcicosis in yellowtail, Seriola quinqueradiata, the cumulative death rate 5 days after infection was 56% for the non-treated group and 0% for the group treated with 100 mg (potency)/kg/day. These results indicate that TBPC will be one of the effective antibiotics for the treatment of enterococcicosis in yellowtail.