"Nature Science Updates", 1 August 2001
Shocked bacteria swap genes.
TOM CLARKE

 Despite its importance in countless Frankenstein movies, most scientists
consider the life-giving properties of lightning to be more theatrical than
actual. But for bacteria at least, bolts from the blue might just be
instrumental in evolution (1).

 Rather than wait for a stormy night, Pascal Simonet and colleagues at
the University of Lyon in France have used a laboratory lightning
generator to show that bacteria in soil can be shocked into incorporating
foreign DNA into their genomes.

 Simonet believes that lightning strikes could play a significant role in
bacterial evolution by increasing the frequency of gene swapping, or
horizontal gene transfer (HGT). This transfer generates genetic diversity
in bacteria moving genes for antibiotic resistance, for example, from
one microbe to another.

Genetic evidence from bacteria suggests that HGT happens far more
frequently than microbiologists can explain. Simonet's team suspect
lightning could be in part responsible since electric fields are widely used
by biologists to make the cell membranes of bacteria permeable to DNA
or other molecules.

They added small rings of DNA - 'plasmids' - coding for antibiotic
resistance to a soil sample containing a strain of the model bacterium
Escherichia coli. This E. coli does not accept DNA in nature, nor does it
commonly colonize soil.

Using a generator that releases bolts of electricity similar to lightning, the
researchers subjected the bacteria/plasmid soil mixture to a laboratory-
scale storm. Some of the bacteria that survived the shocking ordeal
acquired antibiotic resistance, implying that the plasmids had become
part of their genomes. Unshocked bacteria mixed with plasmids in the soil
remained susceptible to the antibiotic.

Soil is teeming with bacteria and DNA remnants from dead organisms, so
'electrotransformation' of bacteria could be "a universal mechanism in
soils", Simonet speculates. His data hint that each lightning strike in
nature could transform up to 10,000 bacteria in some way.

"The reality is probably a much lower frequency, if any," says HGT
expert Wilfried Wackernagel at the University of Oldenburg in Germany.
Although bacterial plasmids, such as those used by the team, do occur
in soil, the majority of free DNA is in strands, he points out.

Strands are taken up much less readily, even when electric fields are
used, Wackernagel argues. But he welcomes the work, saying that the
question of whether lightning could cause DNA to move into bacteria
had been "going around for a long time".

To explore the hypothesis further, Simonet's team is planning field
experiments with typical soil bacteria in areas of France where electrical
storms are frequent.

References

1. Demaneche, S. et al. Laboratory-scale evidence for lightning-mediated
gene transfer in soil. Applied and Environmental Microbiology, 67,
3440 - 3444, (2001).

Oryginal: http://www.nature.com/nsu/010802/010802-7.html

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