Color-Sensing by Phototactic Halobacterium Halobium

Abstract

Halobacterium halobium, a flagellated bacterium which grows in near saturated brine, is phototactic: the cells are attracted to long wavelength visible light (the “red” light response) and repelled by shorter wavelength light (the “blue” light response) (Hildebrand and Dencher, 1975). In the absence of photostimuli, the motility pattern is a three-dimensional random walk resulting from short periods of swimming interrupted by spontaneous reversals which reorient the cell’s swimming direction. Light intensity gradients bias the random walk by modulating the frequency of reversals (Spudich and Stoeckenius, 1979). These spatial gradients are sensed by the cell as time dependent changes in light intensity incurred during its translational motion. Abrupt temporal changes in light intensity mimic the spatial gradients, causing changes in reversal probability, and can be used to assay the cell’s sensory responses.

The type of photosensitive motility response exhibited by bacteria would be called “photoklinokinesis” rather than “phototaxis” by some investigators, who use the latter term in a more restrictive sense (see discussion in Spudich and Stoeckenius, 1979)