With the advent of molecular based methods the description of new taxa is often

With the advent of molecular based methods the

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to fish pathology or science in general is doubtful. With the advent of molecular- based methods, the description of new taxa is often based on minimal phenotypic data, which poses problems for determining reliable diagnostic traits. ANAEROBES Although only two species of anaerobic bacteria, namely Clostridium botulinum and Eubacterium tarantellae, have been implicated as fish pathogens, it is considered likely that detailed biological investigations may reveal that anaerobes cause more widespread problems than has been hitherto realised. In the first place, diagnostic laboratories do not normally use anaerobic methods. Therefore, it is unlikely that isolation of an offending anaerobic pathogen would ever be achieved. Consequently, the cause of disease may not be recognised, or maybe attributed to an aerobic secondary invader. Although there is no evidence that anaerobic pathogens have been missed, there are puzzling cases of mortalities among fish populations for which the aetiological agent has never been isolated. Of course, this could reflect the use of inappropriate methods. It is recognised, however, that anaerobes occur in aquatic sediments (Davies, 1969; Rouhbakhsh-Khaleghdoust, 1975) and in the gastro-intest- inal tract offish (Sakata et al, 1978, 1980; Trust et al, 1979; Austin, 2006), where they would be readily available for initiation of a disease cycle. Trust and colleagues
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48 Bacterial Fish Pathogens recovered Actinomyces, Bacteroides, Fusobacterium and Peptostreptococcus from grass carp, gold fish and rainbow trout. Sakata et al. (1980) described two groups of anaerobes from the intestines of freshwater fish, i.e. ayu, goldfish and tilapia, in Japan. These Gram-negative, non-motile, asporogenous rods were considered to be representatives of the family Bacteroidaceae. It is questionable whether or not these bacteria will be recognised as fish pathogens in the future. Clostridiaceae representative Clostridium botulinum Descriptions of fish-pathogenic Clostridia have tended towards extreme brevity. However, it is clear that the outbreaks of botulism in fish have been caused by predominantly CI. botulinum type E. Clostridium botulinum Cultures comprise anaerobic, chemo-organotrophic. Gram-positive, non-acid-fast rods, of 3.4-7.5 x 0.3-0.7 mm in size, which are motile by means of peritrichous flagella. It is important to note that care should be taken in interpreting Gram-stained smears, because cells may appear Gram-negative with age. Oval, sub-terminally positioned endospores are formed, which have characteristic appendages and exosporia. The cell wall contains diaminopimelic acid. Surface colonies are 1-3 mm in diameter, slightly irregular with lobate margins and raised centres, and translucent to semi-opaque with a matt appearance. Poor to moderate growth occurs in cooked meat broth, but abundant growth occurs in broth containing fermentable carbohydrates at the optimum temperature of 25-30°C. Lecithinase, lipase, neurotoxins and haemolysins, but not catalase, caseinase, H2S, indole or urease, are produced. Gelatinase is not usually produced by CL botulinum type E. Nitrates are not reduced, nor is the Voges Proskauer reaction
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