The ’relative’ cell injury, or conversely the degree of intactness, in a cell population subjected to stress was evaluated using image analysis of the motility of free-swimming cells of Bacillus subtilis, and compared with the cell motion in a non-stressed population. Stress inflicted by the addition of an uncoupling agent of oxidative phosphorylation or heat treatment at 50 degrees C resulted in a decrease in cell motility and an increase in the number of non-motile cells.