Loss of virulence in a protease-deficient mutant of Aeromonas salmonicida

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Abstract
The importance of extracellular protease production in A. salmonicida, the bacterial pathogen of fish furunculosis, was investigated with 4 virulent strains (which were autoagglutinative, hemagglutinative, resistant to fish serum, adhesive to fish tissue culture, protease positive, hemolysin positive, and leukocytolysin positive) and 3 avirulent strains (which were nonagglutinative, nonhemagglutinative, sensitive to serum, nonadhesive, protease positive, hemolysin positive, and leukocytolysin positive). A protease-deficient mutant (NTG-1) was induced by mutagenesis with N-methyl-N''-nitro-N-nitrosoguanidine from virulent strain A-7301, showing protease production, which was common to the 2 strain groups. Strain NTG-1 showed loss of virulence in determinations of LD50, although it remained autoagglutinative, hemagglutinative, serum resistant, adhesive, hemolysin positive, and leukocytolysin positive. A protease fraction separated from extracellular products of strain A-7301 by DEAE-cellulose column chromatography had the capacity to produce skin lesions (furuncles) and high mortality in sockeye salmon. A comparable protein fraction from extracellular products of NTG-1 resulted in no protease activity and no pathological effects on the fish. The avirulent strains were eliminated from rainbow trout in a short time, whereas the virulent strains (including A-7301) were highly infective and proliferated in hosts. NTG-1 preserved its infectively, but fish showed no signs of disease and no mortality. These findings indicate that extracellular protease is a major virulence factor and that protease production in the host is closely implicated in the pathogenesis of fish furunculosis.

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