Molecular characterization and virulence gene profiling of pathogenic Streptococcus agalactiae populations from tilapia ( Oreochromis sp.) farms in Thailand

J Vet Diagn Invest. 2014 Jul;26(4):488-495. doi: 10.1177/1040638714534237.


Streptococcus spp. were recovered from diseased tilapia in Thailand during 2009-2010 ( n = 33), and were also continually collected from environmental samples (sediment and water) from tilapia farms for 9 months in 2011 ( n = 25). The relative percent recovery of streptococci from environmental samples was 13-67%. All streptococcal isolates were identified as S. agalactiae (group B streptococci [GBS]) by a species-specific polymerase chain reaction. In molecular characterization assays, 4 genotypic categories comprised of 1) molecular serotypes, 2) the infB allele, 3) virulence gene profiling patterns ( cylE, hylB, scpB, lmb, cspA, dltA, fbsA, fbsB, bibA, gap, and pili backbone-encoded genes), and 4) randomly amplified polymorphic DNA (RAPD) fingerprinting patterns, were used to describe the genotypic diversity of the GBS isolates. There was only 1 isolate identified as molecular serotype III, while the others were serotype Ia. Most GBS serotype Ia isolates had an identical infB allele and virulence gene profiling patterns, but a large diversity was established by RAPD analysis with diversity tending to be geographically dependent. Experimental infection of Nile tilapia ( Oreochromis niloticus) revealed that the GBS serotype III isolate was nonpathogenic in the fish, while all 5 serotype Ia isolates (3 fish and 2 environmental isolates) were pathogenic, with a median lethal dose of 6.25-7.56 log10 colony-forming units. In conclusion, GBS isolates from tilapia farms in Thailand showed a large genetic diversity, which was associated with the geographical origins of the bacteria.

Keywords: Genetic diversity; Streptococcus agalactiae; molecular characteristic; tilapia; virulence gene profiling..

MeSH terms

  • Animals
  • Aquaculture
  • Bacterial Proteins / genetics
  • Bacterial Proteins / metabolism
  • Cichlids*
  • Fish Diseases / microbiology*
  • Genetic Variation*
  • Genotype
  • Prokaryotic Initiation Factor-2 / genetics
  • Prokaryotic Initiation Factor-2 / metabolism
  • Sequence Analysis, DNA
  • Serogroup
  • Streptococcal Infections / microbiology
  • Streptococcal Infections / veterinary*
  • Streptococcus agalactiae / genetics
  • Streptococcus agalactiae / pathogenicity*
  • Streptococcus agalactiae / physiology*
  • Thailand
  • Virulence


  • Bacterial Proteins
  • Prokaryotic Initiation Factor-2