An extracellular acetylcholinesterase produced by Aeromonas hydrophila is a major lethal toxin for fish.

A hitherto unrecognised lethal toxin from the extracellular products (ECP) of Aeromonas hydrophila is described. The pure toxin was 300 times more toxic than the crude ECP and is the most toxic substance so far described from this bacterium, with a minimum lethal dose of 0.05 micrograms g-1 fish. The toxin had high acetylcholinesterase activity and occurred in native ECP as a monomeric 15.5 kDa polypeptide. The purified toxin had five isoelectric focusing forms ranging from pl 4.45 to 4.70. The ECP of each of six strains of A. hydrophila isolated from fish possessed acetylcholinesterase activity suggesting that the toxin is common in this species. The toxin was not a cytolysin and produced no gross pathology in injected fish. Its enzymic nature, low lethal dose, lack of tissue pathology and its apparent narcotic effect suggest that this toxin may act upon the central nervous system of the fish.