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, email@example.com,
508-793-7752; L. Tsimring, firstname.lastname@example.org, 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.)