salmonicida than with the reference motile aeromonads which according to

Salmonicida than with the reference motile aeromonads

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salmonicida than with the reference motile aeromonads which according to Maclnnes could be attributed to the smaller genome size of Aer. salmonicida. In another investigation, 56-65% binding between Aer. salmonicida and Aer. hydrophila DNA strands was recorded (McCarthy, 1978). These homology values indicate strong genetic relationships between the principal Aeromonas species (Paterson et al, 1980; Belland and Trust, 1988). It is well estabhshed that Aer. salmonicida, as a species, is phenotypically distinct from its motile counterparts. The increasing evidence from genetic and other molecular biology studies pertaining to intrageneric relationships between Aer. salmonicida and other aeromonad species, however, appear to support the retention of the pathogen within the genus. Certainly, the situation is not aided by ongoing manoeuvres in the classification of the motile aeromonads. Therefore, since the arrangement of the genus is in transition, and yet to be definitively
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96 Bacterial Fish Pathogens resolved, we believe that nothing would be gained by the re-classification of Aer. salmonicida. To reiterate, it appears to be the consensus of opinion that the area in need of further work is the intraspecific relationships between typical and atypical isolates. It is anticipated that such work would improve the classification of the genus Aeromonas. Serology Additional approaches to ascertaining the intra- and interspecific relationships of Aer. salmonicida have been adopted. These include serological techniques and bacter- iophage typing. Certainly, the antigenicity of Aer. salmonicida has been the focus of much attention, primarily with a view to its significance in vaccine development. Unfortunately, the early investigations of Williamson (1929) were halted by the persistent agglutination of the strains in saline. This problem was circumvented by Blake and Anderson (1930), who employed complement fixation for the examination of 82 isolates o^ Aer. salmonicida. All of these isolates gave a positive response. Ewing et al. (1961) examined agglutinin absorption with reference to "O" and "H" antigens. They concluded that the 21 strains examined were related to each other, but also to a strain of Aer. hydrophila (O-antigen suspensions prepared using the 21 cultures reacted to ca. 25% of the titre of the O-antiserum prepared with Aer. hydrophila). Other researchers have also found serological homogeneity among strains of Aer. salmonicida, but some degree of cross-reactivity with Aer. hydrophila. For example, common antigens among two Aer. salmonicida strains, as determined by gel- diffusion, and cross-reactions between Aer. salmonicida antiserum and three out of four isolates of Aer. hydrophila, but not of Aer. salmonicida strains and Aer. hydro- phila antiserum, was reported by Liu (1961). Karlsson (1962), using the antigenic properties of a haemolysin from Aer. salmonicida, found no serological differences among the six strains tested. Although these six strains did not cross-react with other aeromonads recovered from humans, it was later established that there were indeed
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