274
late exponential phase of growth. The P2 activity, found in culture supernatants in
the early exponential phase was described as a metalloprotease (molecular
weight = 30–40 kDa) with activity against casein and gelatin. This caseinase activity
was regarded as novel for metalloproteases of Aer. salmonicida subsp . salmonicida.
The second and major protease, P1, appeared in the culture supernatant during late
exponential phase of growth. This protease was regarded as identical to the 70 kDa
serine protease (Lygren et al. 1998 ).
Intramuscular injection of the proteases into brown trout resulted in the development of gross symptoms similar to those occurring in natural outbreaks of furunculosis, i.e. muscle liquefaction along the fl anks adjacent to the lesion and swelling at
the site of injection (Sheeran et al. 1984 ). From these data, Sheeran and co-workers
concluded that their observations differed from those of other workers. They stated
that the absence of signifi cant mortalities, or haemorrhaging of the fi ns and anus in
the experimentally infected fi sh contrasted with the reports of others, suggesting
that, in previous work, enzyme preparations may have contained toxic material
other than proteases. Alternatively, it seems possible that Sheeran used insuffi cient
quantities to achieve the pathological changes in question. However, the latter probability seems to be unlikely insofar as the levels of P1 enzyme injected by Sheeran
et al. ( 1984 ) were equivalent to those of Sakai ( 1978 ), who reported furuncle formation, haemorrhaging and mortalities in kokanee salmon ( Oncorhynchus nerka ) following administration via i.m. injection. Further evidence for the role of protease
production in the pathogenesis of furunculosis was provided by Sakai ( 1977 ), who
reported a reduction in virulence for a proteolytically defi cient mutant strain of Aer.
salmonicida compared with its isogenic wild-type. In his later article, Sakai ( 1985 )
considered a role for proteases in reproduction of the pathogen by making available
small peptides and amino acids from proteolysis. Of course, this would considerably benefi t the nutrition of the pathogen. It should be emphasized that serum from
salmonids is capable of neutralizing lethal doses of proteases (Ellis et al. 1981 ),
possibly through the action of an a-migrating antiprotease (Grisley et al. 1984 ).
Nevertheless, it is apparent that proteases play an important role in the pathogenicity process. Indeed, a serine protease, which was reported to have a molecular
weight of 64 kDa, suppressed the immune response of Atlantic salmon (Hussain
et al. 2000 ). A complication concerns the recovery of a pathogenic non-protease
secreting strain (Tajima et al. 1987 ). Therefore, much work is still required to clarify
the precise mode of action of proteases in the pathogenic process.
The greater susceptibility of brown trout (compared to rainbow trout) to furunculosis has long been recognised (e.g. McCarthy 1975a , b ). An explanation for this
difference has been provided by Ellis and Stapleton ( 1988 ), who found that at low
ratios of exotoxin to serum, brown trout serum considerably enhanced Aer. salmonicida proteolytic activity. Yet at similar ratios, rainbow trout serum demonstrated
some inhibition of the bacterial protease activity. The interpretation of these data is
that during the initial stages of infection, Aer. salmonicida would have greater
potential to multiply in brown trout rather than rainbow trout. Furthermore, Rockey
et al. ( 1988 ) determined that serum from rainbow trout protected the erythrocytes
from the haemolysins of Aer. salmonicida.
5 Aeromonadaceae Representative (Aeromonas salmonicida)
late exponential phase of growth. The P2 activity, found in culture supernatants in
the early exponential phase was described as a metalloprotease (molecular
weight = 30–40 kDa) with activity against casein and gelatin. This caseinase activity
was regarded as novel for metalloproteases of Aer. salmonicida subsp . salmonicida.
The second and major protease, P1, appeared in the culture supernatant during late
exponential phase of growth. This protease was regarded as identical to the 70 kDa
serine protease (Lygren et al. 1998 ).
Intramuscular injection of the proteases into brown trout resulted in the development of gross symptoms similar to those occurring in natural outbreaks of furunculosis, i.e. muscle liquefaction along the fl anks adjacent to the lesion and swelling at
the site of injection (Sheeran et al. 1984 ). From these data, Sheeran and co-workers
concluded that their observations differed from those of other workers. They stated
that the absence of signifi cant mortalities, or haemorrhaging of the fi ns and anus in
the experimentally infected fi sh contrasted with the reports of others, suggesting
that, in previous work, enzyme preparations may have contained toxic material
other than proteases. Alternatively, it seems possible that Sheeran used insuffi cient
quantities to achieve the pathological changes in question. However, the latter probability seems to be unlikely insofar as the levels of P1 enzyme injected by Sheeran
et al. ( 1984 ) were equivalent to those of Sakai ( 1978 ), who reported furuncle formation, haemorrhaging and mortalities in kokanee salmon ( Oncorhynchus nerka ) following administration via i.m. injection. Further evidence for the role of protease
production in the pathogenesis of furunculosis was provided by Sakai ( 1977 ), who
reported a reduction in virulence for a proteolytically defi cient mutant strain of Aer.
salmonicida compared with its isogenic wild-type. In his later article, Sakai ( 1985 )
considered a role for proteases in reproduction of the pathogen by making available
small peptides and amino acids from proteolysis. Of course, this would considerably benefi t the nutrition of the pathogen. It should be emphasized that serum from
salmonids is capable of neutralizing lethal doses of proteases (Ellis et al. 1981 ),
possibly through the action of an a-migrating antiprotease (Grisley et al. 1984 ).
Nevertheless, it is apparent that proteases play an important role in the pathogenicity process. Indeed, a serine protease, which was reported to have a molecular
weight of 64 kDa, suppressed the immune response of Atlantic salmon (Hussain
et al. 2000 ). A complication concerns the recovery of a pathogenic non-protease
secreting strain (Tajima et al. 1987 ). Therefore, much work is still required to clarify
the precise mode of action of proteases in the pathogenic process.
The greater susceptibility of brown trout (compared to rainbow trout) to furunculosis has long been recognised (e.g. McCarthy 1975a , b ). An explanation for this
difference has been provided by Ellis and Stapleton ( 1988 ), who found that at low
ratios of exotoxin to serum, brown trout serum considerably enhanced Aer. salmonicida proteolytic activity. Yet at similar ratios, rainbow trout serum demonstrated
some inhibition of the bacterial protease activity. The interpretation of these data is
that during the initial stages of infection, Aer. salmonicida would have greater
potential to multiply in brown trout rather than rainbow trout. Furthermore, Rockey
et al. ( 1988 ) determined that serum from rainbow trout protected the erythrocytes
from the haemolysins of Aer. salmonicida.
5 Aeromonadaceae Representative (Aeromonas salmonicida)