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A putative heat-labile enterotoxin expressed by strains of Aeromonas media

Department of Microbiology, Institute of Medical Sciences, Banaras Hindu University, Varanasi 221 005, India

Corresponding author: Dr D. V. Singh. Present address: Rajiv Gandhi Centre for Biotechnology, Jagathy, Thiruvananthapuram 695 014, Kerala, India (e-mail: rgcbt{at}md2.vsnl.net.in).

Received 28 April 1999; revised version received 23 Oct. 1999; accepted 5 Jan. 2000.

Abstract

Fifteen isolates of Aeromonas media (seven from diarrhoeal stools, four from water and four from superficial skin ulcers of catfish) were examined for enterotoxin production. Ten of these isolates (six diarrhoeal, one from water and three from fish) caused accumulation of fluid in the initial rabbit ileal loop (RIL) tests. Isolates from diarrhoeal stools and fish caused relatively more fluid accumulation than those from water. Those strains that caused little or no fluid accumulation in the initial experiments became enterotoxin producers after one passage through RILs, regardless of source, and also showed gradual enhancement of fluid outpouring after each subsequent passage. Inocula of c. 1x10⁴ viable cells and 0.25 ml of culture filtrate (CF) caused fluid accumulation similar to that of toxigenic Vibrio cholerae 569B. The enterotoxic factor(s) were inactivated when held at 56°C for 20 min or 65°C for 10 min and showed biological activity over a wide range of pH values. These results suggest that strains of A. media, whether from diarrhoeal stools, water or infected fish, are potentially enterotoxigenic and may have the potential to produce a heat-labile and pH-stable diarrhoeagenic factor in the same way as other known heat-labile and pH-stable enterotoxins.