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Aeromonas caviae, an ubiquitous aquatic organism, has long been considered to be of low pathogenicity, and its virulence mechanisms are still not clearly understood. Twenty-eight A. caviae isolates of clinical origin, most often monomicrobic, were identified in our university hospital over a four year period. Patients, mostly immunocompromised, were: eight diarrhoeal infants, 13 diarrhoeal adults, seven bacteraemic adults. Adults were frequently suffering from underlying intestinal malignancy, hepatobiliary disease, gastrectomy. Virulence factors were investigated. Adherence, studied by use of tissue culture HEp-2 cells, and staining of characteristic lateral flagella, were observed in diarrhoeal strains. Extracellular hemolytic activity was tested on rabbit erythrocytes suspensions at 25 and 37 degrees C. One blood culture isolate showed an important hemolytic activity at 25 degrees C, but none at 37 degrees C. Treatment with furin activated the aerolysin precursor and resulted in significant hemolysis at 37 degrees C, and fluid accumulation in rabbit ileal loops similar to that of A. hydrophila as control. The presence of the hemolysin gene was confirmed in this strain by PCR. In conclusion, A. caviae was shown to be a pathogen isolated from diarrhoea and bacteraemia in immunocompromised patients with malignancies and low gastric acidity as favouring factors. Virulence including the ability to adhere to cells and the production of lateral flagella was observed in diarrhoeal strains. The expression and the production of extracellular hemolytic activity and enterotoxicity at 37 degrees C depended on the activation of the pore forming toxin aerolysin precursor by furin. In vivo the protoxin is probably processed to its mature form by host proteases.