Aeromonads readily grow in waters, particularly if nutrified, to concentrations in excess of total coliforms. Strains of aeromonads can cause gastroenteritis and tissue necrosis. Several suspected virulence factors, such as haemolysins, cytotoxins and enterotoxins may be involved in their pathogenesis. Amongst the thirteen recognised hybridisation groups of Aeromonas, only five species were identified by eight phenotypic characteristics from 339 strains isolated from marine, fresh river or storm waters or from tertiary and sewage/primary effluents. The majority of strains (50%) showed atypical phenotypes, and 24% of 208 randomly selected strains were not identified as aeromonads with a genus specific 16S rDNA-targeted PCR. Most discrepancies occurred with A. schubertii phenotypes, none of which were identified as aeromonads by PCR. Marine waters contained the largest proportion of atypical phenotypes (45/67) of which 60%, compared to <20% for other water sources, were not identified as aeromonads by PCR. A. hydrophila was generally the predominant species identified (93/339), although A. caviae was more prevalent in tertiary treated sewage effluents. Freshwaters contained the largest proportion of aeromonads with haemolysin and/or enterotoxin activity, whereas cytotoxin activity was more prevalent from stormwater isolates. Freshwater strains of A. veronii biotype sobria and A. hydrophila appeared to be the most toxigenic. Furthermore, river sites downstream of sewage effluent release contained more aeromonads than sites immediately upstream of the discharge. Hence, there was a clear positive correlation between freshwater eutrophication and the presence of potentially virulent aeromonads, the majority of which were A. hydrophila and A. veronii which can most rapidly and accurately be identified by PCR.

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