Amino-terminal domain of the El Tor haemolysin of Vibrio cholerae O1 is expressed in classical strains and is cytotoxic.

Previous studies have shown that the classical isolates of Vibrio cholerae possess an 11 bp deletion in the structural gene for the El Tor haemolysin leading to the production of a 27 kDa non-haemolytic truncated product HlyA* compared to the 82 kDa haemolysin, HlyA. These studies were designed to assess whether this truncated product had any biological activity. A KmR cartridge was introduced into the hlyA gene effectively eliminating the haemolysin. This was recombined into the chromosome of a variety of strains and isogenic pairs were examined in a number of systems. These studies suggest that the haemolytic (cytolytic) domain of HlyA resides at the C-terminus and that the N-terminus, which is conserved as HlyA* in classical strains, possesses enterotoxic (cytotoxic) activity. Experiments with the cholera-toxinless vaccine candidate JBK70 and its hlyA::KmR mutant suggest that HlyA* may be responsible for the residual diarrhoea observed in cholera-toxinless vaccine strains.