Phototactic response of aerobically cultivated Rhodospirillum rubrum

Motile cells of aerobically cultivated Rhodospirillum rubrum, containing no detectable bacteriochlorophyll, assembled at a spot of strong light projected through a dark-field condenser. Far-red light was not effective, indicating that bacteriochlorophyll and thus photosynthetic metabolism was not responsible for the phenomenon. Bacteria moving towards the centre of the light spot changed direction less frequently than those moving towards the margin. They also responded to temporal changes in the intensity of light, altering their swimming direction more frequently after a sudden decrease in light intensity than after an abrupt increase.