Enterotoxicity of Vibrio fluvialis

Abstract

Vibrio fluvialis is a halophilic bacterial species which emerged as a diarrhoeal pathogen and has been a human public health hazard all over the world, especially in coastal areas of developing countries and regions with poor sanitation. In this study, we isolated, identified and characterized the enterotoxic and other toxic activities of five V. fluvialis organisms isolated from the environmental sources. Though they are halophilic in nature, however, these organisms were isolated from fresh environmental water sources having 0% salinity from rivers around Dhaka city and ponds from Khulna, and higher salinity in Satkhira area. All the V. fluvialis strains showed growth in 7% salinity and did not produce gas from glucose. In the toxicity assay, live cells and culture filtrates of all five isolates gave positive results in rabbit ileal loop assay indicating their enterotoxicity. Two strains which gave poor results in enterotoxin production initially, showed enhancement of enterotoxicity as judged by increased fluid outpouring after two consecutive passages in rabbit gut indicating that, the enteric environment probably influences toxin production. On the other hand, PCR analysis showed the absence of ctxA gene and other genes like tdh, trh, stx1 and stx2, which are commonly associated with enterotoxicity. The culture filtrate of the isolates showed cytotoxic activities on BHK-21 and HeLa cells lines, gave positive results for haemolysin in blood agar plate and tube methods, and caused paralysis of hind legs of Swiss Albino mice within 2-3 days. The toxic activities shown by culture filtrates of V. fluvilais were increased after concentration of culture filtrates by ammonium sulphate precipitation method. All these results suggested that these environmental V. fluvialis organisms, which lacked ctxA gene, are highly pathogenic. They also lacked other common enterotoxic genes. Although ctxA gene negative, these organisms showed rabbit ileal loop positive reactions, which indicated that they might produce a toxin which is different from the known cholera toxin. Apart from the enterotoxic activity, these organisms also produced neurotoxin, cytotoxin and haemolysin. Therefore, the presence of V. fluvialis organisms in aquatic environment of Bangladesh constitutes a threat to public and environmental health.