Molecular epidemiology of Streptococcus agalactiae isolated from tilapia in Southern China

Streptococcus agalactiae strains were isolated and collected from moribund tilapia in Guangdong,Hainan,and Fujian Provinces,and these isolates were identified by biochemical and molecular biological characterization.In the current study,we compared the phylogenetic relationships among S. agalactiae strains.Furthermore,we compared antibiotic sensitivity,molecular classification and molecular serotype of these strains which were isolated from different years and different geographical regions.Antibiotic sensitivity assays showed that among 20 antibiotics tested,there are 10 antibiotics that had no differences,and the other 10 items showed similar sensitivity.Five most diverse loci(Variable number of tandem repeats,VNTR)were used for the construction of MLVA to evaluate the diversity of these strains.Molecular classification assays showed that all S.agalactiae strains isolated from fish were of the same molecular classification,but the S.agalactiae strain(C918)isolated from dairy cattle was different.Molecular serotype of these isolates using a multiplex PCR reaction,and the results showed that the molecular serotype of S.aglactiae was Ⅰa.However,surface protein genes assays showed that the surface protein of S.agalactiae strains is alpha-C protein except the S.agalactiae(C918)strain.Conclusion:these S. agalactiae isolates isolated from fish are of the same classification by molecular classification assays.Therefore,these results provide a theoretical basis for control of the tilapia diseases and for vaccine development of S.agalactiae in tilapia.