Comparison of the extracellular biological activities of Vibrio anguillarum and Aeromonas hydrophila

In the present study we evaluated the “in vivo” and “in vitro” biological activities of the extracellular products (ECP) from strains of Vibrio anguillarum and Aeromonas hydrophila with different degrees of virulence for fish. The ECP contained moderate levels of protein (0.07 to 0.80 mg/ml), high levels of lipopolysaccharides (0.7 to 3.2 mg/ml) and were strongly toxic for rainbow trout ( Oncorhynchus mykiss Walbaum). The median lethal dose (LD 50) ranged from 1.88 to 3.62 μg ECP protein/g fish ( A. hydrophila isolates) and from 4.72 to 6.36 μg ECP protein/g fish ( V. anguillarum strains). Inoculation of the ECP from non-virulent and low-virulent strains also caused fish mortality. Samples heated at 80 °C/10 min had no effect in fish. Moreover, pathogenic and avirulent strains showed very similar enzymatic and toxic profiles, most of the ECP samples displaying amylolytic, proteolytic, haemolytic, cytotoxic and dermatotoxic activities. In general, these activities were reduced after heating at 56 °C/30 min and totally lost by heating at 80 °C/10 min. Assays with protease inhibitors showed the existence of either or both serine and metaloproteases in the ECP of Vibrio and Aeromonas strains.