and to a lesser extent with Ps fluorescens occurred McCarthy and Rawle 1975

And to a lesser extent with ps fluorescens occurred

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and, to a lesser extent, with Ps. fluorescens occurred. McCarthy and Rawle (1975) concluded that no qualitative differences in serological composition among Aer. salmonicida strains had been demonstrated, but noted that laboratory maintenance of some Aer. salmonicida cultures resulted in progressive loss of serological reactivity, giving negative responses with their antisera. Therefore, they suggested that, when embarking on serological (or vaccination) studies, it is important to include only fresh isolates. Hahnel et al. (1983) used micro-agglutination and double-diffusion precipi- tin tests to study serological relatedness among virulent and avirulent forms of eight isolates of Aer. salmonicida subsp. salmonicida. No serological differences were detected in the virulent isolates, but antigenic differences were observed between the virulent and avirulent form of each culture. Thus, in double-diffusion precipitin tests, the antigens of virulent sonicated cells formed an additional precipitin line when compared with the homogeneous avirulent form. Bacteriophage typing Bacteriophages have been used to study taxonomic relatedness between strains. The first isolation of bacteriophage specific for Aer. salmonicida was made by Todd (1933), although the usefulness for typing purposes was not demonstrated until the work of Popoff (1971a, b). It is considered that phage typing has value in epizootiological studies (Popoff, 1984; Bast et al., 1988; Belland and Trust, 1989). Essentially, the bacteriophages may be divided into three morphological groups and
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98 Bacterial Fish Pathogens ten serological types (Popoff, 1984). Thus, using a set of eight phages, Popoff (1971b) recognised 14 phage types. Also, Paterson et al. (1980) studied phage sensitivity as a means of determining relationships between typical and atypical cultures. Pigmented and achromogenic as well as aggregating and non-aggregating strains showed a high sensitivity to two out of the three bacteriophages. In a further investigation by Rodgers et al. (1981), 27 groups of Aer. salmonicida were defined on the basis of sensitivity patterns to 18 bacteriophage isolates. Significantly, the morphological characteristics of the host bacterium, i.e. whether a rough, smooth or G-phase form, influenced attachment of the bacteriophage. This was apparently attributed to the varying quantities of LPS in the cell wall of the different morphological types. Haemophilus piscium What about Haemophilus piscium, the causal agent of ulcer disease? The name was coined by Snieszko et al. (1950). However, the detailed taxonomic study of Kilian (1976) showed that the organism did not belong in the genus Haemophilus. In particular, the strains did not exhibit requirements for haemin or NAD, which contrasted with the genus description. H. piscium differed from the type species, H. influenzae, in the inability to reduce nitrate or alkaline phosphatase and to grow at 37°C, together with a relatively high G + C ratio of the DNA. Unfortunately, Kilian did not establish the most appropriate taxonomic position of the pathogen.
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