The initial difficulties experienced in culturing the pathogen contributed

The initial difficulties experienced in culturing the

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The initial difficulties experienced in culturing the pathogen contributed signifi- cantly to the uncertainty over its precise taxonomic status. Early work emphasised a few morphological features, namely the presence of small (0.3-1.5 x 0.1-1.0 |im). Gram-positive, asporogenous, non-motile, non-acid-fast rods, which frequently occurred in pairs. Evidence of pleomorphism, metachromatic granules and a "coryneform" appearance (Ordal and Earp, 1956; Smith, 1964) led to the initial.
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64 Bacterial Fish Pathogens tenuous association with the coryneform group of bacteria, namely Corynebacterium. It is interesting to note that the later investigation of Young and Chapman (1978) did not substantiate the "coryneform" morphology. However, transmission electron microscopy of negatively stained cells, obtained from 28-day-old cultures on growth medium, i.e. KDM2 (see Chapter 5), revealed the presence of pleomorphism and intracellular vacuoles/granules (B. Austin, unpublished data). By using FAT on kidney smears from coho salmon, Cvitanich (2004) observed small short rods, termed bar forms because of their staining reaction in FAT, which could not be cultured and were not virulent. Earp (1950) and Ordal and Earp (1956) demonstrated catalase and proteolytic activity, and realised that there was a growth requirement for cysteine. Additional attributes of the organism were slowly reahsed; in particular. Smith (1964) indicated the temperature range of growth, i.e. most rapid at 15°C, slow at 5 and 22°C, and not at all at 37°C, and determined an inability to degrade gelatin. During the period of the late 1970s to early 1980s, a wealth of knowledge was accumulated on Renibacterium. A low genetic diversity among North American isolates has been indicated from multilocus enzyme electrophoresis using 44 enzymes (Starliper, 1996). Thus, from 40 isolates, 21 electrophoretic types were recognised. Grayson et al. (1999) highUghted the inability of conventional systems to differentiate among Renibacterium isolates, and investigated molecular methods that might be useful to identify intraspecific variation. The outcome was the differentiation of isolates by RAPD according to host and geographical location. Renibacterium sahnoninarum Characteristically, Ren. salmoninarum produces cream (non-pigmented), shiny, smooth, round, raised, entire, 2-mm diameter colonies on KDM2 after incubation at 15°C for 20 days. Subclinical infections may lead to two colony types, the smooth colonies described above and a thin film of growth, the latter of which does not develop on SKDM (Hirvela-Koski et ciL, 2006). Old cultures, i.e. 12 weeks, may become extremely granular or crystalline in appearance. Indeed, a transverse section through such colonies will reveal the presence of a few Gram- positive rods embedded in a crystalline matrix. Subculturing at this stage often leads to the development of more crystalline ''colonies". It is thought that the material is principally cystine, which has been precipitated from the medium. For some strains, a uniformly turbid growth occurs in broth, but for others, a sediment may develop. The cell wall peptidoglycan of renibacteria contains D-alanine, D-
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  • Spring '20
  • Bacteria, representative, gram-negative bacteria

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