Chemists in the College of Liberal Arts and Sciences have found a new way to outwit resistance to antibiotics.
By inserting a naturally occurring molecule into an antibiotic-resistant bacterium, the team was able to gradually destroy the machinery responsible for the resistance.
"Multidrug-resistant bacteria are now ubiquitous in both hospital settings and the larger community," writes Paul J. Hergenrother, a professor of chemistry, in a paper that appeared in the Journal of the American Chemical Society. "Clearly, new strategies and targets are needed to combat drug-resistant bacteria."
Antibiotic resistance makes it difficult to fight infection and increases the chance of acquiring one while in a hospital. That, in turn, has led to more deaths from infection, longer hospital stays and a greater use of more toxic and expensive drugs, according to the National Institutes of Health.
Resistance occurs when bacteria develop ways to make themselves impervious, such as by pumping antibiotics out of the cell, preventing them from entering the cell or demolishing them. A common way bacteria develop resistance is by laterally transferring plasmids - pieces of extra-chromosomal DNA - from one bacterium to another. These plasmids contain genetic codes for proteins that make bacteria insensitive to antibiotics.
"Our idea was that if you could eliminate plasmids that make the bacterium resistant, then the bacterium could be sensitive to antibiotics again," Hergenrother says.
The researchers' approach was to use a natural process called plasmid incompatibility. "If there is one plasmid in a cell and another one is introduced, then they compete with each other for resources," Hergenrother said. "One of them wins and the other is eliminated."
With the help of chemistry graduate students Johna C.B. DeNap, Jason R. Thomas and Dinty J. Musk, Hergenrother developed a technique that mimicked plasmid incompatibility by incubating bacteria containing plasmids with a specific compound - in this case an aminoglycoside called apramycin that binds to plasmid-encoded RNA and prevents proper plasmid reproduction.
Apramycin was chosen after numerous potential aminoglycosides - a group of antibiotics effective against gram-negative bacteria - were tested to find those that bind tightly to the target plasmids. Positively charged apramycin bound to negatively charged plasmid-encoded RNA, which allowed apramycin to prevent the actions of the protein that triggers plasmid reproduction. By thwarting that protein, apramycin blocked plasmid replication.
The apramycin treatment was applied to bacterial cultures that were grown for 250 generations. By the end of the experiment, the plasmids no longer were present, making it possible for antibiotics to work.
"This is the first demonstration of a mechanistic-based approach to systematically eliminate the plasmids," Hergenrother says. "Standard antibiotics target the cell wall, but as resistance to antibiotics emerges, there needs to be other targets. We validated that plasmids are a new target for antibiotics."
Further studies are needed to identify whether apramycin is useful against the plasmids occurring in different strains of antibiotic-resistant bacteria. It is possible that other compounds may be needed to target specific plasmids, Hergenrother said. Future studies in his lab will investigate those questions.
The Office of Naval Research, the National Institutes of Health and the Research Corporation, a private Arizona-based foundation that supports basic research in the physical sciences, funded the work.
