STUDIES ON TOBICILLIN, A NEW ANTIBIOTIC DRUG FOR ENTEROCOCCICOSIS IN YELLOWTAIL SERIOLA-QUINQUERADIATA

Tobicillin (TBPC), an ester derivative of penicillin G (PCG), was exam ined as a treatment for enterococcicosis in yellowtail. After 6 h at 2 5 degrees 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 . h) than that for PCG (5.2 mu g/ml . h). In drug sensitivity te sts 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 P CG. On the other hand, the MICs of erythromycin, lincomycin and oxytet racycline showed biphasic distributions that included both resistant a nd sensitive strains. In a preliminary evaluation of the efficacy of T BPC against experimental enterococcicosis in yellowtail, the cumulativ e 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 resul ts indicate that TBPC will be one of the effective antibiotics for the treatment of enterococcicosis in yellowtail.