From watching Bacillus subtilis colonies swimming on moist agar
surfaces, it is natural to think that the bacteria are moving as a fluid.
Presumably, the fluid flows are set into motion by the bacteria swimming. To
determine if there happens to be another force creating the flow, an experiment
was devised and outlined in Colony
Dynamics. The results of the formaldehyde experiments are as follows.
Two film sequences were analyzed. By watching the films as individual frames, the length of time between fumigating the bacteria with formaldehyde and total cessation of movement was calculated. Because small movements were difficult to detect, the exact frame with no continued movement was difficult to distinguish. However, in the first sequence, 206 frames (13.73 seconds) passed until all the cells were still. For the other sequence, 165 frames (11.00 seconds) were required. So, for both runs of the experiment, on the order of ten seconds of formaldehyde exposure eliminates all movement in the colonies.
One question that arises concerns the slight discrepancy in time to cessation of motion. There are two explanations. One is that the cells die immediately, and the fluids require ten seconds to stop moving; this is not likely because momentum is negligible in the system. The other more likely explanation is that approximately ten seconds are required for the cells to metabolize the formaldehyde poison. In latter case, the time to termination of motion represents the inhibition of energy metabolism by the formaldehyde, and the resulting stop of flagella motion.