was homogeneous OHici et al 2000 Atypical isolates of Aeromonas salmonicida

Was homogeneous ohici et al 2000 atypical isolates of

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was homogeneous (O'Hici et al., 2000). Atypical isolates of Aeromonas salmonicida When compared with the so-called motile aeromonads, the description of Aer. salmonicida suggests a very homogeneous group of organisms. Alas, as so often happens in biology, a multitude of exceptions have disturbed the apparent idyllic situation. "Atypical" strains deviate from the classical description of the taxon over a number of biochemical, physiological and genetic properties, e.g. AFLP fingerprints, making typing difficult (Hirvela-Koski et al., 1994; Austin et al., 1998; Wiklund and Dalsgaard, 1998; Dalsgaard et al., 1998; Hoi et al., 1999; Lund et al., 2002). For example, Japanese isolates of so-called atypical Aer. salmonicida were recovered in four groups, which were defined after 16S rDNA sequencing. There was not any host specificity with these groups (Yamada et al., 2000). The most common reasons for describing isolates as "atypical" are: lack of weak or slow pigment production (Nakatsugawa, 1994; Koppang et al., 2000); catalase negativity (Kaku et al., 1999); oxidase negativity (Wiklund and Bylund, 1993; Wiklund et al., 1994; Wiklund and Dalsgaard, 1995; Pedersen et al, 1994, 1996a; Kaku et al, 1999); nutritional fastidiousness, i.e. for blood or blood products (Austin, 1993);
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90 Bacterial Fish Pathogens slow growth, i.e. >5 days to obtain visible colonies (Austin, 1993; Kaku et ai, 1999); different hosts from salmonids, i.e. cyprinids (e.g. Austin, 1993; Kaku et al, 1999) and marine fish, including shotted hahbut (Eopsetta grigorjewi; Nakatsu- gawa, 1994), dab, plaice and flounder {Platichthys flesus) (Wiklund et al, 1994; Wiklund and Dalsgaard, 1995), common wolf fish (Anarhichas lupus) (Hellberg et ai, 1996), turbot (Scophthalmus maximus) (Pedersen et ai, 1994), greenling (Hexagrammos otakii), Japanese flounder (Paralichthys olivaceus) and Schlegel's black rockfish (Sebastes schlegeli) (lida et al, 1997), where the disease is often ulceration. With the last mentioned example, the justification for describing the isolates as atypical was based on the host rather than the characteristics of the cultures. One of the earliest indications that aberrant strains occurred was provided by Smith (1963), who examined six isolates of non-pigmented cultures, which were clustered as Group I from a numerical taxonomy study. These organisms were related at the 75.6% similarity level to typical pigment-producing isolates. Smith (1963) proposed a separate new species for Group I, i.e. with the specific epithet achromo- genes, although the recommendation was not adopted. A second, non-pigmented group was recognised by Kimura (1969a) as Aer. salmonicida subsp. masoucida. This subspecies differed from typical strains on account of indole production, Voges Proskauer reaction, H2S and lysine decarboxylase production, and fermentation of sucrose (Table 7.1). In the eighth edition of Sergey's Manual of Determinative Bacteriology, Schubert (1974) regarded these non-pigmented isolates as Aer. salmo- nicida subsp. achromogenes and Aer. salmonicida subsp. masoucida, respectively.
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