In contrast rainbow trout were relatively resistant to the effects of all four

In contrast rainbow trout were relatively resistant

This preview shows page 323 - 325 out of 594 pages.

with this fraction survived. In contrast, rainbow trout were relatively resistant to the effects of all four fractions, insofar as no mortalities resulted. However, administra- tion of fraction II resulted in the development of characteristic furuncle-like lesions at the inoculation site. Fractions I, III and IV did not cause any obvious pathology. The results of Cipriano et al. (1981) supported the previous findings of Sakai (1977) who, on the basis of work with crude ECP preparations, considered that a protease was the most pathogenic substance produced by Aer. salmonicida. The effects ascribed to proteolytic activity by Sakai (1977) were analogous to those noted by Cipriano et al. (1981) for fraction II. In fact, the muscle necrosis and degeneration of connective tissue associated with furunculosis indicates proteolytic enzyme activity.
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304 Bacterial Fish Pathogens Yet, Fyfe et al. (1986) recorded that protease preparations were less effective than equivalent amounts of ECP (with similar amount of proteolytic activity) at causing lesions, i.e. furuncles, following i.m. injection of juvenile Atlantic salmon. This team identified three major components with molecular weights of 70kDa (a serine pro- tease; Elhs et al, 1997), 56kDa (a haemolysin) and 100 kDa (unidentified protein) in the ECP (Fyfe et al., 1987a); the first mentioned of which was produced in greater quantities after incubation for 18 h at 25°C compared with 125 h at 10°C (Fyfe et al., 1987b). Haemolysin production was similar at both temperatures, but 10-fold more of the 100 kDa protein was produced at the lower temperature. Proteases, as prime candidates for exerting a significant role in disease patho- genesis, have aroused substantial interest as a research topic. Indeed, a variety of investigators have performed detailed analyses, and suggested heterogeneity among isolates. In particular, Gudmundsdottir (1996) described 6 protease groups, but this information might have greater value for taxonomy than an understanding of pathogenicity. Thus, by examining 5 typical and 25 atypical isolates, it was deter- mined that the proteases produced by the type strains of Aer. salmonicida subsp. achromogenes (this produced a metallo-caseinase = AsaPl) and Aer. salmonicida subsp. salmonicida were different from those of the fresh isolates. Moreover, all the typical isolates belonged to one protease group with proteolytic activities com- parable with PI and P2 proteases, whereas the atypical cultures were different. With the exception of three atypical oxidase-negative isolates, which secreted a protease reminiscent of PI, the others produced metallo-gelatinase. Ten of the atypical isolates produced AsaPl (Gudmundsdottir, 1996). Shieh and MacLean (1975) purified a proteolytic enzyme, which was determined to have a molecular weight of 11 kDa, and an optimum pH range of 8-11. Because the enzyme was inhibited by PMSF, these workers concluded that it was a serine protease. Mellergaard (1983) also isolated and purified a proteolytic enzyme (molecular weight = 87.5 kDa; optimum pH of 9.0), as did Tajima et al. (1984),
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