264
infi ltration at the foci of infection. Mackie and Menzies ( 1938 ) confi rmed the production of a leucocytolytic substance, as did Field et al. ( 1944 ), who determined the
absence of leucocytosis by performing repeated blood counts on experimentally
infected carp. Perhaps, a more signifi cant fi nding of their study, however, was the
rapid decline in blood sugar levels resulting in hypoglycaemic shock, which was
suffi cient in some instances to cause acute mortalities. They suggested that the
hypoglycaemic shock was the outcome of rapid utilisation of blood glucose by the
multiplying pathogen. Regarding virulence mechanisms of Aer. salmonicida, Griffi n
( 1953 ) theorised that leucocidin production in vivo by Aer. salmonicida would
account for the observations by previous workers that marked cytolytic tissue necrosis did not seem to be accompanied by leucocytic infi ltration. Another aspect of Aer.
salmonicida pathogenicity, which eventually proved to be extremely important, was
discussed by Duff ( 1937 ). He reported a loss in pathogenicity among strains after 6
or more months of maintenance on artifi cial culture in the laboratory. The loss was
accompanied by a change in the appearance of colonies on nutrient agar from glistening, convex and translucent to strongly convex, distinctly opaque and creamcoloured. Because of such observations, Duff further investigated this phenomenon
of dissociation into different colony types. Subsequently, he discovered that dissociation could be induced by culturing the pathogen in nutrient broth with the addition of either 0.25 % lithium chloride or 0.1 % phenol. Use of this procedure gave
rise to several distinct colony forms. One of these resembled the original stock culture, a second corresponded to the ‘new’ type, and a third was intermediate between
the other two forms. The colonies resembling those of the original stock culture
were described as opaque, strongly convex, cream-coloured and friable, whereas
the new colony form appeared translucent, slightly convex, and a bluish-green in
colour with a butyrous consistency. When the two different colony types were inoculated intraperitoneally into goldfi sh, the blue-green, translucent dissociant caused
the deaths of the fi sh and was accompanied by lesions typical of the disease. In
contrast, the original type of colony did not adversely affect fi sh, which survived for
the 30-day duration of the experiment without any signs of illness. Thus, Duff concluded that the cream-opaque form which produced friable colonies was nonpathogenic, and more stable on prolonged storage. Duff designated this colony type
as ‘rough’. The ‘smooth’ form (i.e. the blue-green-translucent dissociant, which
produced butyrous colonies on agar media) was pathogenic, but less stable in prolonged storage. In the subsequent study, Duff ( 1939 ) also reported the presence of
an extra antigen in the rough strains. Although Duff ( 1937 ) was the fi rst worker to
report the ability of Aer. salmonicida to dissociate into several distinct colony types
with differences in pathogenicity, a phenomenon which is now widely accepted, it
is curious that he ascribed pathogenicity to the smooth colony type. This is in contrary to the view currently held that the rough colony type is, in fact, virulent.
Interestingly, the Furunculosis Committee had also reported a variation in colony
morphology among isolates (it may be assumed that these corresponded to the
rough and smooth variants) but contended that this phenomenon was not accompanied by a difference in virulence. It is regrettable that this initial confusion over
dissociation occurred, preventing an earlier realisation of its signifi cance. In fact,
5 Aeromonadaceae Representative (Aeromonas salmonicida)
infi ltration at the foci of infection. Mackie and Menzies ( 1938 ) confi rmed the production of a leucocytolytic substance, as did Field et al. ( 1944 ), who determined the
absence of leucocytosis by performing repeated blood counts on experimentally
infected carp. Perhaps, a more signifi cant fi nding of their study, however, was the
rapid decline in blood sugar levels resulting in hypoglycaemic shock, which was
suffi cient in some instances to cause acute mortalities. They suggested that the
hypoglycaemic shock was the outcome of rapid utilisation of blood glucose by the
multiplying pathogen. Regarding virulence mechanisms of Aer. salmonicida, Griffi n
( 1953 ) theorised that leucocidin production in vivo by Aer. salmonicida would
account for the observations by previous workers that marked cytolytic tissue necrosis did not seem to be accompanied by leucocytic infi ltration. Another aspect of Aer.
salmonicida pathogenicity, which eventually proved to be extremely important, was
discussed by Duff ( 1937 ). He reported a loss in pathogenicity among strains after 6
or more months of maintenance on artifi cial culture in the laboratory. The loss was
accompanied by a change in the appearance of colonies on nutrient agar from glistening, convex and translucent to strongly convex, distinctly opaque and creamcoloured. Because of such observations, Duff further investigated this phenomenon
of dissociation into different colony types. Subsequently, he discovered that dissociation could be induced by culturing the pathogen in nutrient broth with the addition of either 0.25 % lithium chloride or 0.1 % phenol. Use of this procedure gave
rise to several distinct colony forms. One of these resembled the original stock culture, a second corresponded to the ‘new’ type, and a third was intermediate between
the other two forms. The colonies resembling those of the original stock culture
were described as opaque, strongly convex, cream-coloured and friable, whereas
the new colony form appeared translucent, slightly convex, and a bluish-green in
colour with a butyrous consistency. When the two different colony types were inoculated intraperitoneally into goldfi sh, the blue-green, translucent dissociant caused
the deaths of the fi sh and was accompanied by lesions typical of the disease. In
contrast, the original type of colony did not adversely affect fi sh, which survived for
the 30-day duration of the experiment without any signs of illness. Thus, Duff concluded that the cream-opaque form which produced friable colonies was nonpathogenic, and more stable on prolonged storage. Duff designated this colony type
as ‘rough’. The ‘smooth’ form (i.e. the blue-green-translucent dissociant, which
produced butyrous colonies on agar media) was pathogenic, but less stable in prolonged storage. In the subsequent study, Duff ( 1939 ) also reported the presence of
an extra antigen in the rough strains. Although Duff ( 1937 ) was the fi rst worker to
report the ability of Aer. salmonicida to dissociate into several distinct colony types
with differences in pathogenicity, a phenomenon which is now widely accepted, it
is curious that he ascribed pathogenicity to the smooth colony type. This is in contrary to the view currently held that the rough colony type is, in fact, virulent.
Interestingly, the Furunculosis Committee had also reported a variation in colony
morphology among isolates (it may be assumed that these corresponded to the
rough and smooth variants) but contended that this phenomenon was not accompanied by a difference in virulence. It is regrettable that this initial confusion over
dissociation occurred, preventing an earlier realisation of its signifi cance. In fact,
5 Aeromonadaceae Representative (Aeromonas salmonicida)