Epidemiological studies and diagonosis of Streptococcus dysgalactiae isolated farmed fish

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The Lancefield serological group C Streptococcus sp. was isolated from cultured amberjack, Seriola dumerili Risso and yellowtail, Seriola quinqueradiata Temminck and Schlegel that had been immunized with Lactococcus garvieae commercial vaccines in Japan. The isolated bacteria were gram-positive cocci, auto-aggregated in saline, morphologically long chains in growth medium, catalase negative and α-haemolysis on blood agar. An almost complete gene sequence of the 16S rDNA of isolates (n=2) was determined and compared to that of bacterial strains in the data base. The isolates were identified to Streptococcus dysgalactiae based on the results of the 16S rDNA sequence, the bacteriological properties and the Lancefield serological grouping. The oligonucleotide primers designed with specifity for the 16S-23S rDNA intergenic spacer region of S. dysgalactiae amplified a gene from all the fish isolates as well as the type strains of α-haemolytic S. dysgalactiae subsp. dysgalactiae ATCC430738 and β-haemolytic S. dysgalactiae subsp. equisimilis ATCC35666, but not those of S. equiATCC33398, Lactococcus garvieaeATCC43921 and L. garvieae KG94 08. The severe lesion (a necrotic lesion of the caudal peduncle) observed on experimentally infected fish was similar to that seen on naturally infected fish. Further attention is required to distinguish the Lancefield group C Streptococcal infection from L. garvieae infection in S. dumerili and S. quinqueradiata in Japan.