Effect of temperature on hemolysin production in Aeromonas spp. isolated from warm and cold environments

The effect of growth temperature on the hemolysin production in 97 Aeromonas isolates was studied. Only 9% of the isolates obtained from low-temperature sources such as refrigerated foods, drinking water and trout aquaculture were able to produce high hemolysin titers (titer greater than 128) at 37 degrees C compared with 65% of the isolates from warm sources such as human clinical cases and warm water aquaculture (p less than 0.001). In contrast, 61% of the low-temperature isolates were producing high hemolysin titers at 10 degrees C while only 24% of the isolates from high temperature sources produced high titers at this temperature (p less than 0.05). Hemolytic activity could decrease markedly during longer (48 h) incubation at 37 degrees C. Some of the filtrates from growth at 37 degrees C and 10 degrees C were tested for enterotoxin-like activity in the suckling mice test. Of the culture filtrates produced at 37 degrees C, 40% of A. hydrophila were positive for enterotoxin-like activity, irrespective of hemolysin titer. Of A. sobria 88% of the filtrates with high hemolytin titers and none of the low-titer filtrates were enterotoxigenic. Due to a high mortality rate of mice injected with filtrates produced at 10 degrees C the enterotoxic effect of these filtrates could not be established. Autoagglutination after boiling was observed in 31% of the clinical isolates and 2% of the strains of environmental origin.