THE IDENTIFICATION AND HUMAN HEALTH SIGNIFICANCE OF ENVIRONMENTAL AEROMONADS

Aeromonads readily grow in waters, particularly if nutrified to concen trations in excess of total coliforms. Strains of aeromonads can cause gastroenteritis and tissue necrosis. Several suspected virulence fact ors, such as haemolysins, cytotoxins and enterotoxins may be involved in their pathogenesis. Amongst the thirteen recognised hybridisation g roups of Aeromonas, only five species were identified by eight phenoty pic characteristics from 339 strains isolated from marine, fresh river or storm waters or from tertiary and sewage/primary effluents. The ma jority of strains (50%) showed atypical phenotypes, and 24% of 208 ran domly selected strains were not identified as aeromonads with a genus specific 16S rDNA-targeted PCR. Most discrepancies occurred with A. sc hubertii phenotypes, none of which were identified as aeromonads by PC R, Marine waters contained the largest proportion of atypical phenotyp es (45/67) of which 60%, compared to <20% for other water sources, wer e 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 enterotoxi n activity, whereas cytotoxin activity was more prevalent from stormwa ter isolates. Freshwater strains of A. veronii biotype sobria and A. h ydrophila appeared to be the most toxigenic. Furthermore, river sites downstream of sewage effluent release contained more aeromonads than s ites immediately upstream of the discharge. Hence, there was a clear p ositive correlation between freshwater eutrophication and the presence of potentially virulent aeromonads, the majority of which were A. hyd rophila and A. veronii which can most rapidly and accurately be identi fied by PCR.