Effect of heat stress on cell activity and cell morphology of the tropical rhizobium, Sinorhizobium arboris

The effect of heat stress on the growth, physiological state, cell activity and cell morphology of the tropical Sinorhizobium arboris strain HAMBI 2190 was studied. The cells were chromosomally tagged with the firefly luciferase gene, luc. Since the bioluminescence phenotype is dependent on cellular energy reserves it was used as an indicator of the metabolic status of the cell population under various heat conditions. Variations in the numbers and lengths of growth phases between individual cultures indicated that the growth pattern at 40°C was disturbed compared to growth at 37 or 28°C. In addition, the cell morphology was changed radically. The number of culturable cells and the luciferase activity declined when the cultures were incubated at 40°C. By contrast, under all conditions studied, the cells could be stained with 5-(and 6-)sulfofluorescein diacetate, indicating esterase activity. This demonstrated that although the culturability and cellular energy reserves decreased considerably during heat stress, a majority of the of S. arboris cell population maintained basal enzyme activity.