chains exists insofar as trout suffer heavy mortalities whereas Mozambique

Chains exists insofar as trout suffer heavy

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chains exists, insofar as trout suffer heavy mortalities whereas Mozambique bream (Sarotherodon mossambicus), banded bream (Tilapia sparramanii), carp (Cyprinus carpio) and largemouth bass (Microterus salmoides) do not (Boomker et al, 1979). It has been established that challenge with low-virulence isolates or low doses of high-virulence isolates together with cell-free culture supernatants are sufficient to establish infection (Kimura and Kusuda, 1979). The toxic activity of supernatants was further researched, and two fractions were demonstrated to have a significant effect on pathogenicity (Kimura and Kusuda, 1982). These were recovered in Todd- Hewitt broth after incubation at 30°C for 48 h. The fraction, although not toxic by oral administration (presumably the compounds were digested), produced damage, i.e. exophthalmia and petechial haemorrhages, following percutaneous injection of yellowtails. Co-infection of Str. iniae with aquabirnavirus has led to higher mortal- ities in Japanese flounder (Pakingking et al, 2003). Evidence has been presented that a cell capsule may be involved with the resist- ance of Gram-positive cocci in chains to opsonophagocytosis in yellowtail (Yoshida et ai, 1997). This view was reinforced by Miller and Neely (2005), who when using capsular mutants showed that the capsule was indeed important for the virulence of Str. iniae. Again, an effect on phagocytosis was reported. Similarly, capsules have been reported in Lactococcus garvieae, with encapsulated cultures being more
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Pathogenicity 285 virulent (Barnes et ai, 2002) and less efficient at fixing complement compared with non-encapsulated isolates (Barnes and Ellis, 2004). Non-encapsulated cultures were more susceptible to normal rainbow trout serum than capsulated isolates (Barnes et al, 2002). Two capsular types have been found among Lactococcus garvieae, one of which produces a well-developed capsule, whereas the second demonstrates a micro- capsule which contains fimbrial-type components projecting from the cell surface (Ooyama et ai, 2002). Also, polysaccharide capsules have been found on Str. iniae (Barnes et ai, 2003). The pathogen produces a cytolysin with haemolytic traits, which is a functional homologue of streptolysin S. Expression of this cytolysin is necessary for local tissue necrosis, but not to bacteraemia (Fuller et al, 2002). When grown in serum, this streptococcus expresses surface factors that are capable of binding to trout immunoglobulin by the Fc region (= crystallisable fragment of the immuno- globuHn) (Barnes et ai, 2003a). A range of isolates from fish, a dolphin and humans produced apoptosis and/or necrosis in tilapia non-specific cytotoxic cells and tilapia- continuous cell lines (Taylor et al, 2001). Only serotype II strains entered, multipHed and survived in pronephros phagocytes (leading to apoptosis) for >48h. This is relevant because it was estimated that ~70% of the bacteria contained in blood during sepsis were located within phagocytes, which suggests a preferred intracellular existence (Zlotkin et al, 2003).
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  • Spring '20
  • Bacteria, representative, gram-negative bacteria

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