A simplistic approach for the diagnosis of columnaris has involved the obser

A simplistic approach for the diagnosis of columnaris

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A simplistic approach for the diagnosis of columnaris has involved the obser- vation of wet preparations, prepared from lesion material, which were supposed to demonstrate the presence of "column"-Hke masses of bacteria (Snieszko, 1958b). Undoubtedly, use of this technique led to mis-diagnosis with other myxobacteria. However, a recent development, which is meeting with some success, is the microscopic examination of spleen squashes from fish considered to be suffering with RTFS (Sarti et al., 1992). Here, bacteria can be readily seen in the infected animals.
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200 Bacterial Fish Pathogens BACTERIOLOGICAL EXAMINATION OF TISSUES Although detailed in Chapter 5, it is worth re-iterating that by use of a comparatively small range of media—a couple of incubators adjusted to 15°C and 20-25°C, an anaerobic jar, and standard aseptic techniques—the majority of the bacterial fish pathogens may be readily recovered as pure culture growth. It should be emphasised, however, that in some asymptomatic "carrier" states, e.g. BKD, only a scant number of colonies may develop. Nevertheless, in the majority of clinical cases of disease, it should be possible to obtain dense growth. Tissues to be sampled Generally, examination should be made of any damaged tissues. Experience has taught us that it is always advisable to include a kidney sample, which often permits the most satisfactory isolation of the pathogen. Quite simply it is adequate to sample the material by means of swabbing. The swabs should then quickly be used to inoculate the bacteriological media. Culturing Aeromonas salmonicida Diagnosis of Aer. salmonicida is readily achieved by culturing techniques, usually on TSA or BHIA (the preferred medium in the view of many scientists) in which case "typical" isolates produce a characteristic brown, diffusible pigment. Also, CBB may be employed as a differential medium (Markwardt et ai, 1989). However, as a primary isolation medium, CBB appears to be less sensitive and gives a poor recovery of the pathogen compared with BHIA (B. Austin, unpublished data). A special case for diagnosis BKD Confusion surrounds which of the available methods is most suitable for detecting and thus diagnosing the presence of BKD. Essentially, the opposing opinions include those who favour serology and those who recommend cultivation. Gene probes are also entering the armoury of the diagnostician (Etchegaray et al, 1991). It is difficult for most microbiologists to determine which of the alternative approaches is correct. Historically, diagnosis of BKD was achieved histologically, by the presence of Gram- positive cocco-bacilH in kidney tissue. However, the rehability was impaired by the presence of melanin granules and other morphologically similar bacteria (Chen et ai, 1974). A subsequent derivative has been a histochemical technique using Lillie's allochrome to stain glycogen in the bacteria (Bruno and Munro, 1982).
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  • Spring '20
  • Bacteria, representative, gram-negative bacteria

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