Abstract
Antimicrobial susceptibility of Streptococcus parauberis against nine drugs were investigated in 64 strains isolated from Japanese flounder Paralichthys olivaceus at different culture areas of western Japan, and resistance genes were determined by PCR and Southern blot hybridization. Five of the tested strains belonging to S. parauberis serotype I (44 strains) were resistant to oxytetracycline and erythromycin and were found to possess both tet(S) and erm(B) genes. Minimum inhibitory concentrations (MICs) of oxytetracycline and erythromycin were 128 and 512 μg/mL, respectively. Southern blot hybridization analysis showed that the tet(S) gene was encoded on an 11-kb conjugative plasmid, while the erm(B) gene resided on the chromosomal DNA. All of the examined S. parauberis serotype II strains (20 strains) were tetracycline-resistant (MIC = 32 μg/mL) and were found to possess tet(M) gene. Both int (integrase) and xis (excisase) genes were also detected in all of the serotype II strains, suggesting the presence of Tn916-related element in these strains. These results present a threat of difficulty in treatment of S. parauberis infection by tetracyclines, especially those caused by serotype II strains.
