Experiments challenge genetic theory.
For more than 40 years, microbiologists have held that the only bacteria able to survive environmental upheavals, such as abrupt temperature shifts or food shortages, are those that have changed, or mutated, before encountering the stress. But recent experiments may prompt scientists to revise this view. Researchers at the Harvard School of Public Health in Boston say they have shown that bacteria can somehow adopt genetic traits in response to a particular environment, then pass on these acquired characteristics to their offspring.
This field has long remained dormant, says one of the researchers, in part because it is so complicated.
In one set of experiments, reported in the Sept. 8 NATURE, the scientists began with a population of Escherichia coli bacteria incapable of metabolizing the sugar lactose. When such lac.sup.- E. coli mutate to lac.sup.+., they acquire the ability to survive in an environment whose only source of sugar is lactose. The researchers introduced lac.sup.- E. coli to lactose and, not surprisingly, verified established genetic rules: The lac.sup.- E. coli that happened to have mutated to lac.sup.+ before encountering lactose survive. But using complex statistical analysis, the scientists also observed that a small number of bacteria mutate from lac.sup.- to lac.sup.+ when they encounter lactose.
The experiments revealed that the response was specific to the lac gene; other genes did not mutate in response to the lactose. The researchers conclude that bacterial mutations arise spontaneously and randomly and that bacteria can mutate in a more purposeful manner, to adapt to a particular environment. In other words, the two genetic views are not necessarily mutually exclusive, they believe.
John Cairns, who led the study, says he does not have the final explanation for the unexpected results, although he suggests that yet-undiscovered molecular mechanisms may provide an answer, and proposes a theory along those lines. On the other hand, evolutionary biologist Barry Hall of the University of Connecticut in Storrs, who will report findings supporting Cairns' in an upcoming issue of GENETICS, says scientists in the field do not yet have enough information to propose such theories. Another possibility is that certain experimental factors, such as the culture medium changing the rate of mutation, might account for the results, suggests population geneticist Bruce Levin of the University of Massachusetts in Amherst.
Whatever the explanation, says Levin, "these experiments are important because they force scientists to reconsider an issue thought to be closed."
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|Date:||Sep 10, 1988|
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