In vitro and in vivo comparisons of virulent and attenuated strains of the salmonid fish pathogen Renibacterium salmoninarum

A 57 kilodalton protein known as p57 or MSA is one possible virulence factor of Renibacterium salmoninarum, the causative agent of a disease of salmonid fishes known as bacterial kidney disease (BKD). The p57 antigen is the predominant protein on the bacterial cell surface and may be important for host cell attachment and thus virulence. A non-agglutinating strain of R. salmoninarum, MT 239, does not produce surface-extractable p57 and is relatively avirulent in rainbow trout, Oncorhynchus mykiss . Results of an injection challenge suggested that MT 239 is also less virulent in chinook salmon, Oncorhynchus tshawytscha. The current study investigates differences between the attenuated MT 239 strain and a virulent strain, ATCC 33209 so as to further understand the virulence mechanisms of R. salmoninarum. Fish exposed to MT 239 had significantly lower mortality and showed no clinical or histological sips of BKD when compared to fish exposed to the same levels of ATCC 33209. Western blot analyses with both strains demonstrated significantly lower levels of p57 in whole cell fractions of MT 239 compared to ATCC 33209. Similar levels of p57 were observed in the supernatants of broth cultures, suggesting lower total p57 expression in MT 239. A genetic comparison revealed that both strains have two copies of the msa gene encoding p57. DNA sequencing of both msa genes in both strains revealed no differences within and between the two strains. Northern blot experiments examining msa transcription also revealed no difference between the two strains. A molecular comparison using random amplified polymorphic DNA (RAPD) detected no difference between ATCC 33209 and MT 239. The difference in virulence between the strains observed in vivo may be due to reduced levels of cell-associated p57 and total p57 expression in MT 239.