Molecular Serotyping and Antimicrobial Susceptibility of Streptococcus Agalactiae Isolated from Fish in China

Streptococcus agalactiae (group B streptococcus, GBS) is a major pathogenic bacterium frequently associated with septicemia and meningoencephalitis in fish. In this study, we used molecular typing and the disc diffusion method to determine serotypes and antimicrobial susceptibilities of 28 S. agalactiae isolates from diseased farmed fish (tilapia, Schizothorax prenanti and Schizopygopsis pylzovi) in China. We found that the 28 S. agalactiae isolates belonged only to two serotypes (Ia and III), with serotype Ia being significantly more prevalent than serotype III. Antimicrobial susceptibility results showed that all of the isolates were susceptible to vancomycin, cephalexin and florfenicol. The S. agalactiae isolates examined showed resistance to erythromycin (42.9%), penicillin (35.7%), clindamycin (28.6%), enrofloxacin (35.7%), tetracycline (32.1%), gentamicin (28.6%) norfloxacin (28.6%), ofloxacin (14.3%) and doxycycline (7.14%), but the serotype III isolates showed higher proportions of antibiotic resistance than serotype Ia isolates. The macrolide resistance gene (ermB) was detected in all the 28 isolates and the tetracycline resistance gene (tetM) was detected in 78.6% isolates. All quinolone-resistant S. agalactiae isolates carried mostly double point mutations in DNA with inferred amino acid substitutions including the change of Ser-81 to Leu in the product of gyrA and Ser-79 to Tyr in the product of parC. Our study is a valuable reference for effective antibiotic application of S. agalactiae infections of farmed fish in China, and illustrates the importance of better use of antibiotics to prevent the widespread emergence of resistance S. agalactiae.