Ultraviolet Prompts Bacterial Trek

Evidence of the progressive depletion of ozone in our upper atmosphere has prompted several studies of the effects of increasing ultraviolet (UV) radiation on biological systems. In general, such studies focus on repair mechanisms at the cellular level.

A research group at Clark University in Worcester, Massachusetts has now looked at larger scale behavior in a bacterial colony exposed to elevated UV levels. The researchers (A. Kudrolli, akudrolli@clarku.edu, 508-793-7752; L. Tsimring, ltsimring@ucsd.edu, 858-534-0816) were surprised to find that colonies of microscopic bacteria migrated to form macroscopic, ring-shaped distributions in response to UV exposure (see figure).

The study began with evenly distributed colonies of the common soil bacteria Bacillus subtilis grown on nutrient-rich media. The bacteria launched their migration toward the edges of the colonies after the UV light was turned on. Initially, there seemed little benefit to the patterns because the UV intensity was uniform across the colony. Indeed, when the radiation was switched off the bacteria returned to the evacuated area inside the ring, confirming that the growing media was still hospitable.

What benefit could the bacteria gain through their travels? The researchers propose that the added stress due to UV light causes the bacteria to become more sensitive to slightly increased levels of waste products built up in the central portion of the colony, and that the bacteria near the colony edges emit chemical attractants that lure their kin to pristine media regions.

The swarming migration of bacteria under UV light is a remarkable example of a complex reaction to stress in a biological system. It is particularly important in the light of continuing threats to the ozone layer that protects bacteria and humans alike from harmful radiation. (A. M. Delprato, A. Samadani, A. Kudrolli, and L.S. Tsimring, Physical Review Letters, 8 October 2001.)