Fifteen Aeromonas isolates from various human infections and nine isolates from polluted water were identified as either Aeromonas hydrophila or Aeromonas sobria and examined for cytotoxigenicity, enterotoxigenicity, adherence to epithelial cells, and other virulence-associated factors, including proteases, lipases, elastases, and hemolysins. Two groups cf organisms (I and II) were distinguishable based on differences in median lethal doses in mice and cytotoxicity for Y-1 adrenal cells. Group I clinical and environmental strains had median lethal doses of <107 colony-forming units, were cytotoxic, frequently possessed several virulence-associated factors, and had lysine decarboxylase-positive or Voges-Proskauer-positive phenotypes or both. Piliation of Aeromonas was associated strongly with ability to adhere to human buccal cells, and these characteristics were associated with group I strains. Group Il clinical and environmental strains had median lethal doses of '107 colony-forming units, were not cytotoxic, and usually were lysine decarboxylase negative or Voges-Proskauer negative or both. Clinical strains in group II exhibited enterotoxigenicity, which was not detected in group Il environmental strains. A. sobria was more frequently associated with human infections; 13 of the 15 clinical strains were A. sobria, and 2 were A. hydrophila. On the other hand, the majority of the environmental strains (seven of nine) were A. hydrophila.Aeromonas species are ubiquitous, waterborne microorganisms that have been implicated repeatedly as the causative agents of clinical illnesses, often serious, ranging from gastrointestinal and wound infections to septicemia (4,8,9,12,18,25,26). In compromised patients these illnesses may be fatal. Whereas a small proportion of humans, approximately 1%, are carriers of Aeromonas (26), infections frequently are acquired as a direct result of exposure to water harboring this microorganism (8,9,12,27).
