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New study overturns orthodoxy on how macrophages kill bacteria
Date:4/27/2009

derstand the role of an enzyme is to see what happens when it is absent, so the researchers looked at mutant S. typhimurium that had the genes for SodA, SodB, or both enzymes, deleted. Deleting the gene for SodA seemed to make no difference, but the SodB mutants were less able to survive and cause disease in a mouse. The double mutants were even more impaired. They were much, much less likely to survive in the mouse than bacteria with only the SodB gene missing. These findings "offer genetic proof" that both enzymes "are involved in the same process," Slauch said.

The fact that the bacterial mutants were less likely to survive in a mouse did not prove, however, that the missing enzymes were protecting the bacteria from ROS generated in the mouse macrophages, Slauch said.

"You get the same result if you grow these mutants in the laboratory in aerobic conditions," he said.

Furthermore, the SodA/SodB mutant bacteria were profoundly weakened even in a mouse that was unable to produce the potent ROS superoxide in its macrophages. These results suggest that the superoxide dismutases in the bacterial cytoplasm are most likely protecting the bacterium from its own, naturally occurring ROS, Slauch said.

In contrast, deleting the gene encoding the periplasmic superoxide dismutase, SodC, conferred the same defect regardless of whether the cytoplasmic SodA/SodB were present or absent, showing that its function is independent of the cytoplasm.

Moreover, strains lacking SodC were impaired only in the presence of superoxide produced in macrophages; there was no impairment in laboratory media or in mice lacking the ability to make superoxide.

This suggests that the superoxide and other reactive oxygen species are not making it from the macrophage into the bacterial cytoplasm, Slauch said.

"We conclude from all this data that the most sensitive target of ROS in the macrophages lies outside the cytoplasm," Slauc
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Contact: Diana Yates
diya@illinois.edu
217-333-5802
University of Illinois at Urbana-Champaign
Source:Eurekalert  

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