The precise nature of the virulence mechanism of V anguillarum has prompted

The precise nature of the virulence mechanism of v

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The precise nature of the virulence mechanism of V. anguillarum has prompted some excellent work. With the advent of random genome-sequencing, a strain (H775-3) was examined and 40 genes which may well be related to virulence identi- fied, of which 36 genes were considered to be novel to V. anguillarum, and included genes for capsule biosynthesis, enterobactin, haemolysin, flagella, LPS biosynthesis, pilus and protease (Rodkhum et al., 2006). The highUght of the early studies was the
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330 Bacterial Fish Pathogens discovery that bona fide strains of serogroup Ol of the pathogen contained a virulence plasmid, which was associated with an iron uptake system expressed under iron- Hmited conditions (Crosa et al, 1977, 1980; Crosa, 1980; Wolf and Crosa, 1986; Chen , 1996). This plasmid, designated pJMl and of 67kb, has been fully sequenced (Di Lorenzo et al, 2003), and is always present in virulent isolates (Pedersen et al, 1996b,c, 1997b) and may be included on a transposon-like structure (Tolmasky and Crosa, 1995), but absent from those of low virulence. Conversely, the pJMl plasmid has been found in some avirulent isolates (Pedersen et ai, 1997b). Yet, virulence may be attenuated by curing this plasmid (Crosa et al, 1980) or by deleting three plasmid- encoded gene products (Singer et al, 1991). The role for pJMl concerns specifying an iron-sequestering mechanism, i.e. the low molecular weight siderophore anguibactin for which the precursor is chromosome-mediated 2,3-dihydroxybenzoic acid (Chen et ai, 1994), and specific iron transport proteins, of which the angR protein (this is regulated by the regulatory gene angR [Salinas and Crosa, 1995] which has been reported as similar to bacteriophage P22 [Farrell et al, 1990]) acts as a positive regulator of anguibactin biosynthesis and the transcription of the iron transport genes Fat A and FatB (Actis et ai, 1995; Chen et ai, 1996). Also, V. anguillarum has a plasmid-encoded histamine decarboxylase gene angH, which is essential for the biosynthesis of anguibactin (Barancin et al, 1998). The overall effect is that the system enables the bacterial cell to compete for available iron in the fish tissues. Two OMP have been designated as OM2 (molecular weight = 86 kDa) and OM3 (molecular weight = 79 kDa). The siderophore and OM2 are coded by plasmid pJMl, whereas OM3 is a function of chromosomal involvement. The basic mechanism involves diffusion of the siderophore into the environment, and the formation of iron complexes that attach to OM2, presumably leading to transport of the iron into the bacterial cell (Crosa and Hodges, 1981; Crosa et al, 1983; Tolmasky and Crosa, 1984; Actis et ai, 1985; Mackie and Birkbeck, 1992). Thus, invading bacteria may multiply in the host by scavenging successfully for the iron that is bound by high- affinity iron-binding proteins, such as transferrin, lactoferrin and ferritin. These are present in the serum, secretions and tissues, respectively (Bullen et al, 1978). Toranzo et al. (1983a) compHcated the issue by publishing data that showed that virulent strains, obtained from striped bass, did not contain plasmids. Yet, all the isolates
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  • Spring '20
  • Bacteria, representative, gram-negative bacteria

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