Virulence characteristics of Aeromonas spp. in relation to source and biotype

The significance of Aeromonas spp. as potential water-borne enteric pathogens in Tasmania, Australia, an area with a mild climate and comparatively low year-round water temperatures, was investigated in view of the reported marked peak of Aeromonas-associated gastroenteritis in the summer and the apparent influence of temperature on levels of potentially pathogenic species in water supplies. Biochemical characteristics and virulence-associated properties--exotoxin production (hemolysin, enterotoxin), ability to grow at 43 degrees C, and possession of pili--were determined for 105 Tasmanian isolates of Aeromonas spp.; 43 isolates were from clinical specimens (greater than 75% diarrhea associated) and 62 were from water. Current classification schemes were evaluated for these isolates. A. sobria comprised 35% of the clinical isolates and 16% of the water isolates, A. hydrophila comprised 56 and 79%, and A. caviae comprised 9 and 5%. A total of 42% of the clinical isolates and 15% of the environmental isolates were enterotoxigenic (by the suckling mouse assay); these levels were significantly lower than those found in warmer environments. The majority (74%) of enterotoxigenic isolates were A. sobria. Enterotoxin-producing isolates possessed three or more of the following properties. They were Voges-Proskauer positive, did not hydrolyze arabinose, were positive for lysine decarboxylase, were able to grow at 43 degrees C, and produced large amounts of hemolysin (titer, greater than 128). Thus, the biochemical scheme proposed by Burke et al. (V. Burke, J. Robinson, H.M. Atkinson, and M. Gracey, J. Clin. Microbiol. 15:48-52, 1982) for identifying enterotoxigenic isolates appears to have widespread applicability. Environmental enterotoxigenic isolates possessed numerous pili, but these appeared to be lost once infection was established, as a similar isolates from patients with diarrhea were poorly piliated.