Aeromonads are ubiquitous gram-negative bacteria found in aqueous environments. Some members of the genus are pathogenic for fish, reptiles and cows. In humans, Aeromonas infection is mainly associated with grastrointestinal diseases, but in immuno-compromised individuals infection can lead to septicemia and meningitis (Austin et al. 1996). Aeromonas secretes a variety of virulence factors amongst which aerolysin is the best characterized. Using marker exchange mutagenesis, aerolysin was demonstrated to be required not only for the establishment but also for the subsequent maintenance of systemic infections associated with the bacterium (Chakraborty et al. 1987). Furthermore, specific neutralizing antibodies to aerolysin have been detected in animals surviving Aeromonas infection.