Molecular characterization of Streptococcus agalactiae in diseased farmed tilapia in China

A severe outbreak of streptococcosis in cultured tilapia (Oreochromis niloticus) caused by Streptococcus agalactiae (Group B streptococcus, GBS) has occurred annually in Southern China during the past years, causing significant economic losses. Little is known about the genetic characteristics of the tilapia S. agalactiae that is prevalent in these areas. A total of 51 GBS isolates from tilapia were collected from 27 farms located in distinct geographic regions. The genetic characteristics of GBS were analyzed by MLST, MLVA, serotyping, and PCR screening of mobile genetic elements, genetic marker, and virulence-related genes. The results showed that all of S. agalactiae tilapia isolates have the genotype, Ia-ST7-bac-bca-fbsA-sip-cfb-IS1381-IS861-GBSil-ISSag2, which suggests a low level of genetic diversity and sharing of a recent and similar origin and a low level of genetic differentiation under similar environment selective pressures. Compared with the control, significant differences were detected between the tilapia strains and the S. agalactiae strains of human and bovine origin. Phage typing successfully differentiated all 51 tilapia GBS isolates into two distinct molecular types (type A and B). Interestingly, the phage type of the tilapia GBS isolates was also observed to shift from type A to type B, with the year 2010 as the turning point. Type B GBS isolates are currently prevalent in tilapia populations in China, and should thus be the target strains to detect and develop prophylactic–therapeutic measures. The genetic data obtained in the present study will be helpful in the determination of the epidemiology of tilapia streptococcosis internationally. •Similar selective pressures led to little genetic differentiation on Tilapia GBS.•A very useful typing tool for GBS strains, phage typing, was found.•A genetic shift of tilapia GBS isolates prevalent in the endemic area was detected.