By Katherine Harmon
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The superbug MRSA (methicillin-resistant Staphylococcus aureus) has provoked fear in doctors and patients alike because it is endowed with genetic characteristics that make it impervious to many antibiotics, and it can be deadly to boot. Less well known, however, is another class of bacteria that also resist antibiotics, but for reasons that have puzzled scientists. These bugs cause stubborn infections in ears and urinary tracts and post-surgical wounds, even though, from their genetic profiles, they should be perfectly good targets for antibiotics.
Researchers are now starting to figure out how these bacteria withstand antibiotic treatment: by exploiting the same traits that have helped them endure environmental stressors. Two new research papers, published Friday in Science, show how bacteria use their ability to withstand prolonged periods without food or exposure to reactive oxygen to also fight off antibiotics. Knowing what these defenses are could lead to new ways of making existing therapies more effective.
Scientists do not yet fully understand how antibiotics work on the molecular level, but they think that the drugs are effective in part by introducing reactive oxygen to bacterial cells, which damages key cellular structures.
Evgeny Nudler, a biochemist at the New York University School of Medicine, and his colleagues have unraveled details about how bacteria protect themselves from such "oxidative stress." They produce hydrogen sulfide, which, in combination with nitric oxide (a typical by-product of bacterial metabolism) seems to protect bacteria from antibiotic assaults. Nudler’s team found three enzymes that are responsible for triggering the production of this gas in Staphylococcus aureus, Escherichia coli and other bacteria. "It was a major surprise," he says. "If you treat cells with antibiotics, you see that they start producing more [hydrogen sulfide] right away."
Bacteria that are starved for nutrients can also turn their weakened state to their advantage in warding off antibiotics, according to the second paper. Scientists have known that nutritionally deprived bacteria are better able to resist the chemical blows dealt by antibiotics. This reaction, known as stringent response, is common across bacterial species. The question was whether the bacteria were just hunkering down when faced with low levels of nutrients or were more actively defending themselves.
Dao Nguyen, a microbiologist at McGill University in Montreal, and her colleagues investigated the behavior of strains of Pseudomonas aeruginosa, a relatively common bug that can cause infections in the urinary tract, kidneys or lungs, that were isolated from patients who had chronic infections. They found that when the bacteria were not getting enough nutrients, they showed signs of a stringent response. "As the bacteria sense starvation… [they] produce an alarm signal called (p)ppGpp," Nguyen explained in a Science podcast. "And this allows the cells to regulate a vast number of genes, which then allows it to better adapt and survive in response to starvation and stress."
To test whether stringent response could also be protecting the bacteria from antibiotics, the researchers created a mutant strain that lacked such an alarm. Indeed, antibiotics were much more effective against those bacteria strains that could not turn on their stringent response. The pattern held up in mice as well. When mice infected with bacteria lacking the response were given antibiotics, their infections cleared up and the mice survived. "With normal, wild type bacteria, the mice would die even if you treated them with the antibiotics," Nguyen said in the podcast.
Researchers are still learning more about just how, molecularly, these different responses are triggered. "These studies together show that bacteria have clever antioxidant strategies to counter the oxidative damage generated by antibiotics," notes James Collins, an investigator at the Howard Hughes Medical Institute, who also co-authored an essay about the two new papers in the same issue of Science. Both of the research teams suggest ways in which these newly described mechanisms could boost the strength of drugs we already have.
"Perhaps you could find a way to exploit this starvation response in such a way that if you disrupt the stringent response somehow, you could sensitize the bacteria to currently available antibiotics," Nguyen said. And if the enzymes that trigger the bacteria's creation of protective hydrogen sulfide in Nudler's study could be disabled, the bugs could be rendered more susceptible to drugs. He and his team are using high-throughput screening to find small molecules that inhibit the enzymes. They have already found a couple candidates that seem capable of taking out one of the enzymes.
But just learning more about the complexity of these well-adapted bacteria also suggests that, "our battles with bugs may be tougher than we thought," Collins cautions.