277
Insofar as no appreciable differences were recorded between the levels of ECP from
virulent (possessing an A-layer) and avirulent (no A-layer) cells, it appears that
these compounds are not virulence determinants.
Hastings and Ellis ( 1985 ) reported that there was a marked variation in the production of haemolysins and proteases among different strains of Aer. salmonicida.
In their study, four isolates, recovered from Atlantic salmon in Scotland, produced
caseinase and gelatinase. Both of these enzymes were inhibited by PMSF (a serine
protease inhibitor) and to a lesser extent by EDTA (a divalent metal ion chelator).
This fi nding indicated that both enzyme activities could be attributed to a single
serine protease, which was dependent upon divalent cations for activity. In contrast,
an achromogenic isolate that was obtained from Iceland did not produce detectable
quantities of haemolysin or gelatinase. It is noteworthy that the caseinase from this
isolate differed from that of the Scottish strains, insofar as it resembled a metalloprotease. Hastings and Ellis ( 1985 ) noted that this enzyme appeared to be unique to
fi sh pathogens. It is not known, however, if other achromogenic strains of Aer. salmonicida share similar properties regarding their ECPs. Nevertheless, it is relevant
to note that yet another strain, recovered from the USA, differed from the Scottish
isolates insofar as it lacked both caseinase and gelatinase activity in the
ECP. Moreover, its haemolysin production was notably lower. Hence, it seems that
there is a marked variation in the nature of the precise components of the ECPs from
different strains of the pathogen. Thus, there may be some variation in the modes of
pathogenesis.
Yet another factor with a potential role in virulence and pathogenicity is the leucocytolytic component of the ECP. Although this component was recognised in the
1930s, many years passed before detailed study ensued. Thus, Klontz et al. ( 1966 )
reported leucopenia in rainbow trout, following injection of either viable cells or a
saline soluble extract of the culture. On the basis of these results it was hypothesised
that a leucocytolytic compound was responsible for the limited leucocyte activity
observed in infected fi sh. This group did not comment, however, on the biochemistry
of the leucocytolytic compound. Nevertheless, this aspect was examined by Fuller
et al. ( 1977 ), who deduced that the compound was a glycoprotein, which was distinct from the endotoxin, i.e. LPS, as previously studied by Ross ( 1966 ), Anderson
( 1973 ) and Paterson and Fryer ( 1974a , b ). The leucocytolytic factor was present in
the supernatants from broth cultures; moreover, virulent strains produced more than
the avirulent counterparts. Furthermore, the glycoprotein was cytolytic for leucocytes in vivo, and produced a pronounced leucopenia when injected intravenously
into adult rainbow trout. In addition, the factor appeared to enhance pathogenicity,
presumably by increasing susceptibility of the host. This opinion was reached after
experiments in which small coho salmon were inoculated with the leucocytolytic
compound in combination with live Aer. salmonicida (approximately a quantity suffi cient to achieve a LD 50 ). The result was that 36/40 fi sh succumbed, in contrast to
the death of only 14/40 animals injected with just the pathogen. So, the conclusion
was reached that the glycoprotein constituted a virulence factor of Aer. salmonicida.
However, the results of Cipriano et al. ( 1981 ) were not in accord with the fi ndings
of Fuller’s team. Essentially, Cipriano and colleagues deduced that there was no
Aeromonas salmonicida
Insofar as no appreciable differences were recorded between the levels of ECP from
virulent (possessing an A-layer) and avirulent (no A-layer) cells, it appears that
these compounds are not virulence determinants.
Hastings and Ellis ( 1985 ) reported that there was a marked variation in the production of haemolysins and proteases among different strains of Aer. salmonicida.
In their study, four isolates, recovered from Atlantic salmon in Scotland, produced
caseinase and gelatinase. Both of these enzymes were inhibited by PMSF (a serine
protease inhibitor) and to a lesser extent by EDTA (a divalent metal ion chelator).
This fi nding indicated that both enzyme activities could be attributed to a single
serine protease, which was dependent upon divalent cations for activity. In contrast,
an achromogenic isolate that was obtained from Iceland did not produce detectable
quantities of haemolysin or gelatinase. It is noteworthy that the caseinase from this
isolate differed from that of the Scottish strains, insofar as it resembled a metalloprotease. Hastings and Ellis ( 1985 ) noted that this enzyme appeared to be unique to
fi sh pathogens. It is not known, however, if other achromogenic strains of Aer. salmonicida share similar properties regarding their ECPs. Nevertheless, it is relevant
to note that yet another strain, recovered from the USA, differed from the Scottish
isolates insofar as it lacked both caseinase and gelatinase activity in the
ECP. Moreover, its haemolysin production was notably lower. Hence, it seems that
there is a marked variation in the nature of the precise components of the ECPs from
different strains of the pathogen. Thus, there may be some variation in the modes of
pathogenesis.
Yet another factor with a potential role in virulence and pathogenicity is the leucocytolytic component of the ECP. Although this component was recognised in the
1930s, many years passed before detailed study ensued. Thus, Klontz et al. ( 1966 )
reported leucopenia in rainbow trout, following injection of either viable cells or a
saline soluble extract of the culture. On the basis of these results it was hypothesised
that a leucocytolytic compound was responsible for the limited leucocyte activity
observed in infected fi sh. This group did not comment, however, on the biochemistry
of the leucocytolytic compound. Nevertheless, this aspect was examined by Fuller
et al. ( 1977 ), who deduced that the compound was a glycoprotein, which was distinct from the endotoxin, i.e. LPS, as previously studied by Ross ( 1966 ), Anderson
( 1973 ) and Paterson and Fryer ( 1974a , b ). The leucocytolytic factor was present in
the supernatants from broth cultures; moreover, virulent strains produced more than
the avirulent counterparts. Furthermore, the glycoprotein was cytolytic for leucocytes in vivo, and produced a pronounced leucopenia when injected intravenously
into adult rainbow trout. In addition, the factor appeared to enhance pathogenicity,
presumably by increasing susceptibility of the host. This opinion was reached after
experiments in which small coho salmon were inoculated with the leucocytolytic
compound in combination with live Aer. salmonicida (approximately a quantity suffi cient to achieve a LD 50 ). The result was that 36/40 fi sh succumbed, in contrast to
the death of only 14/40 animals injected with just the pathogen. So, the conclusion
was reached that the glycoprotein constituted a virulence factor of Aer. salmonicida.
However, the results of Cipriano et al. ( 1981 ) were not in accord with the fi ndings
of Fuller’s team. Essentially, Cipriano and colleagues deduced that there was no
Aeromonas salmonicida