Proteases of the Aeromonas hydrophila complex: identification, characterization and relation to virulence in channel catfish, Ictalurus punctatus (Rafinesque)

. Traditional biochemieal techniques and a stain to detect proteases in polyacrylamide gels were used to identify and partially characterize three proteases, P1, P2 and P3, produced by Aeromonas hydrophila strain Ah 22. P1 was found to be a heat‐labile serine protease with an optimum pH of 7·5, while P2 is a heat‐stable metalloprotease with an optimum pH of 8·0, and P3 is a moderately heat‐stable metalloprotease with peak activity beween pH 7 and 11. A comparison of 17 other strains of the A. hydrophila complex indicated that four produced P1, P2 and P3. Two strains produced just P1 and P3; one produced only P3; six produced two different serine proteases, P2a and P2b; and two produced a number of uncharacterized proteases. Virulence studies in age‐0 + channel catfish indicated no correlation between either quantitative or qualitative protease production and virulence.