Shell disease syndrome in the edible crab, Cancer pagurus--isolation, characterization and pathogenicity of chitinolytic bacteria.

Chitinolytic bacteria are believed to be the primary aetiological agents of shell disease syndrome in marine crustaceans. The disease principally results from the breakdown of their chitinous exoskeletons by the shell disease pathogens, but pathogenicity may also manifest internally should a breach of the carapace occur. The current study looks at the pathogenicity of a number of bacteria (predominantly from the genus Vibrio) isolated from the edible crab, Cancer pagurus. All chitinase-producing bacteria investigated were capable of growth in a minimal medium consisting of chitin powder from crab shells, but differed in their speed of growth and nature of chitinolytic activity, suggesting that they have different roles within the lesion community. Two isolates (designated I4 and I7) were chosen for studies on internal pathogenicity, which included the effect of the pathogen on crab tissues, the ability of the host to remove the bacteria from circulation and the antibacterial activity of crab blood. Initially, I4 was rapidly removed from circulation, but began to reappear in the blood after 24 h. By 100 h, 100% of crabs were moribund. The septicaemic effects of the isolate were reflected in the low levels of its killing by blood-cell lysate and serum. By contrast, I7 was only slowly removed from circulation and caused the rapid mortality of all crabs in <3 h. A large decline in the number of circulating blood cells following injection of I7 was mirrored by an accumulation of these cells in the gills. Initial experiments suggest that the death of the crabs following injection with I7 may be caused by toxic extracellular bacterial products that exert their effects on the blood cells and nervous system of the crabs.