Microbiological studies of Streptococcus sp. biovar 1 (synonym of Enterococcus seriolicida and Lactococcus garvieae), pathogenic for rainbow trout (Oncorhynchus mykiss), and related species of bacteria

A specific Gram-positive coccus, which was given the epithet Streptococcus sp. biovar 1, was first isolated from diseased farmed rainbow trout (Oncorhynchus mykiss) in Tasmania in 1988. The preliminary identification was based on the morphological, physiological and biochemical characteristics of the Tasmanian isolates. However, subsequent studies in Japan and Tasmania indicated that these organisms were more closely related to the genus Enterococcus, especially the fishpathogenic bacterium known as Enterococcus seriolicida, than to the genus Streptococcus. Further investigation has indicated that these fish pathogenic isolates are identical to the species Lactococcus garvieae isolated from cattle. In December 1993, when this study commenced, the source of Streptococcus sp. biovar 1 (as it was then known) in Tasmania was unknown, so an environmental survey of potential sources was undertaken with the view to determining the initial source of the organism and providing a basis for future control of the disease. The 61 organisms isolated during this survey, as well as ten Australian isolates of Streptococcus sp. biovar 1, two strains of Enterococcus seriolicida and two strains of Lactococcus garvieae, were characterised phenotypically using morphological, physiological, biochemical and immunological techniques. Random amplified polymorphic DNA polymerase chain reaction (RAPD-PCR) studies were then undertaken on the ten Australian isolates of Streptococcus sp. biovar 1, two strains of Enterococcus seriolicida, one strain of Lactococcus garvieae and 18 of the environmental isolates to investigate the possibility of any genetic similarity. A specific Gram-positive coccus, which was given the epithet Streptococcus sp. biovar 1, was first isolated from diseased farmed rainbow trout (Oncorhynchus mykiss) in Tasmania in 1988. The preliminary identification was based on the morphological, physiological and biochemical characteristics of the Tasmanian isolates. However, subsequent studies in Japan and Tasmania indicated that these organisms were more closely related to the genus Enterococcus, especially the fishpathogenic bacterium known as Enterococcus seriolicida, than to the genus Streptococcus. Further investigation has indicated that these fish pathogenic isolates are identical to the species Lactococcus garvieae isolated from cattle. In December 1993, when this study commenced, the source of Streptococcus sp. biovar 1 (as it was then known) in Tasmania was unknown, so an environmental survey of potential sources was undertaken with the view to determining the initial source of the organism and providing a basis for future control of the disease. The 61 organisms isolated during this survey, as well as ten Australian isolates of Streptococcus sp. biovar 1, two strains of Enterococcus seriolicida and two strains of Lactococcus garvieae, were characterised phenotypically using morphological, physiological, biochemical and immunological techniques. Random amplified polymorphic DNA polymerase chain reaction (RAPD-PCR) studies were then undertaken on the ten Australian isolates of Streptococcus sp. biovar 1, two strains of Enterococcus seriolicida, one strain of Lactococcus garvieae and 18 of the environmental isolates to investigate the possibility of any genetic similarity. In vivo pathogenicity to rainbow trout (Oncorhynchus mykiss) was also studied to compare the virulence of the Australian isolates of Streptococcus sp. biovar 1 with that of Lactococcus garvieae. This was performed as two separate experiments. The first experiment was to establish the LD50 of one of the Tasmanian isolates, including examination of histopathology to evaluate the tissue tropism of the organism. The second experiment was to passage Lactococcus garvieae isolated from a case of bovine mastitis through rainbow trout. Two techniques, direct kidney tissue transfer and culture and re-isolation, were used to see if the organism could produce disease. The results of all these studies showed that the Australian isolates (Streptococcus sp. biovar 1) were closely related to, if not identical with, both Lactococcus garvieae and organisms previously named Enterococcus seriolicida. The results also showed that none of the isolates from the environment were phenotypically or genotypically the same as the Australian isolates or Lactococcus garvieae. Therefore, Streptococcus sp. biovar 1 should probably be reclassified as Lactococcus garvieae, and, because it is known to be the cause of streptococcosis in many countries around the world, it must now be considered a major fish pathogen worldwide.