Growth characteristics of motile Aeromonas spp. isolated from different environments

The growth of 80 strains of motile Aeromonas spp. derived from environments with temperatures above 25 degrees C and below 15 degrees C, respectively, were examined at five temperatures (5 degrees C, 10 degrees C, 25 degrees C, 37 degrees C and 44 degrees C) and four salt levels (0.05%, 2%, 4% and 6% NaCl). Sixty-one strains were further examined at two pH levels (pH 7.3 and pH 5.3). All strains grew at 25 degrees C and 10 degrees C with the majority of the isolates proliferating from approx. 10(2) to approx. 10(7) cfu/ml within 1 and 3 days, respectively. In contrast, there were significant differences in the proportion of isolates able to grow at 5 degrees C and 37 degrees C depending on the temperature of their source of isolation. The ecological background of the organisms thus influences their thermal growth range and their ability to proliferate at body temperature, a highly significant factor in infective disease. At 25 degrees C and pH 7.3, all strains grew in 0.05% NaCl, 96% grew in 2% NaCl, 96% grew in 2% NaCl while few grew in broth containing 4% or 6% NaCl. Lowering the pH to 5.3 with lactic acid caused a marked increase in the lag phase at 25 degrees C and prevented growth of a large number of isolates at suboptimal conditions. Thus, none of the isolates from warm environments and only 8% of the isolates from cold environments grew at this pH at 5 degrees C. The observed differences in growth optima between strains from different environments are discussed in relation to food- or waterborne infection.