Three distinct classes of antibiotics kill bacterial cells with reactive oxygen species
By Melissa Lee Phillips | August 31, 2007
All three major classes of antibiotics share a single mechanism for killing bacterial cells, reports this week's Cell. Although these drugs initially have different effects on bacterial cells, they all converge on a pathway that kills cells by generating highly reactive free radicals.
The results suggest new ways of improving antibiotic effectiveness, the authors say.
"This is one of those neat, unpredictable findings," said Scott Singleton of the University of North Carolina at Chapel Hill, who was not involved in the study. "It's really not just a linear extension of what we knew before."
Drugs that kill bacteria, called bactericidal antibiotics, are grouped into three classes, depending on how the drug damages bacterial cells. One class inhibits DNA replication and repair, another inhibits protein synthesis, and the third prevents cell-wall turnover. "Prior thinking was that cell death arose principally from those interactions and that each [class] acted differently," said senior author James Collins of Boston University.
Earlier this year, Collins's group reported that one class of antibiotics induces production of reactive oxygen species, especially hydroxyl radicals, which cause bacterial cell death by inducing oxidative DNA and protein damage. To see if the other two classes might damage bacteria in the same way, the researchers -- led by Michael Kohanski and Daniel Dwyer, both of Boston University -- exposed Escherichia coli to one of each of the three classes of bactericidal antibiotics.
Using a dye that fluoresces in the presence of hydroxyl radicals, they found that all three antibiotics produced free radicals in E. coli. They also tested antibiotics in another bacterium, Staphylococcus aureus, with the same results. However, when the researchers conducted the same experiment with five bacteriostatic antibiotics, which inhibit bacterial growth without killing the cells, they found no increase in hydroxyl radical levels.
To show that the hydroxyl radicals were responsible for bacterial cell death, the researchers blocked radical formation in one experiment and treated the cells with an antioxidant in another. In both cases, stopping free radical activity increased survival of bacteria treated with any of the three types of antibiotics.
The authors also found that core components of bacterial metabolism -- including the tricarboxylic acid (TCA) cycle and the respiratory electron transport chain -- are required to generate these hydroxyl radicals, showing that all three antibiotics generate hydroxyl radicals through the same mechanism.
Their results do not discount the established mechanisms through which each antibiotic class acts, Collins told The Scientist. But, "in addition to these separate mechanisms, there is a common one that's being induced in all cases."
"It is really quite new and quite startling," said Graham Walker of the Massachusetts Institute of Technology, who was not involved in the work. "This is certainly not what the textbooks say" about antibacterial mechanisms, he said.
The findings help explain results from several studies over the past few decades, Singleton added. Previous studies discovered, for example, that disabling the bacterial DNA damage response can increase the effectiveness of two types of antibiotics. Also, studies found that antibiotic-resistant bacterial mutants had dysfunctions in proteins that generate reactive oxygen species.
Researchers may be able to develop drugs to improve current antibiotics, either by increasing hydroxyl radical production in bacteria or by blocking the bacteria's own damage response systems, Collins said. That might "make antibiotics more effective, which would allow them to work at a lower dose," Singleton agreed.
However, the antibiotic concentrations used in the study were relatively low, said Kim Lewis of Northeastern University in Boston, who was not involved in the work, and it's possible that other mechanisms of cell death might be more important at higher drug concentrations. "But clearly what they discovered seems to be an important component of death," he said.
Melissa Lee Phillips
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