What happens when the solution is also the problem? That’s a question scientists have been grappling with for years as antibiotic-resistance continues to spread—and grow—on a global scale. And researchers at the University of Washington have just added another wrinkle.
In a paper published in Nature Ecology & Evolution, the UW researchers report that, for a bacterial pathogen already resistant to an antibiotic, prolonged exposure to that antibiotic not only boosts its ability to retain its resistance gene, but also makes the pathogen more readily pick up and maintain resistance to a second antibiotic and become a multidrug-resistant strain. In recent years, multidrug-resistant bacteria—or bacteria that are resistant to two or most antibiotics—have proven especially difficult to treat. This category includes methicillin-resistant Staphylococcus aureus, better known as MRSA.
Co-senior authors Ben Kerr (University of Washington) and Eva Top (University of Idaho) chose an unusual target for their antibiotic resistance experiments: plasmids. Many scientists dismiss plasmids as a target since they can interfere with processes inside a bacterial cell. However, they are a fundamental part of bacterial communities and they boast a unique feature—they can be passed on both vertically (between parent and offspring) and horizontally (between unrelated cells and species). The ability to move horizontally was a distinguishing factor for this specific research study.
In public health settings, like hospitals and nursing homes where MDR can run rampant, horizontal transfer allows antibiotic resistance to travel from one bacterial species to another in mere minutes. For example, given the right exposure, a single bacterial cell can “collect” multiple plasmids in one afternoon, rending it resistant to three or four different antibiotics. Meanwhile, a few hours ago, it was only resistance to one specific antibiotic.
For the experiment, the research team worked with E.coli containing a tetracycline-resistance plasmid and Klebsiella pneumoniae cells containing a chloramphenicol-resistance plasmid. Both bacteria were first grown in an antibiotic-free environment, before being exposed to strains of antibiotics.
After nine days in an antibiotic-free environment, neither E.coli nor Klebsiella showed loyalty to their plasmids. Less than 50% of Klebsiella held on to their respective plasmid, while only 20% of E.coli kept theirs. Conversely, after growing each bacteria for 400 generations in an antibiotic-exposed medium, more than half of E. coli and Klebsiella cells held on to their respective plasmid. Surprisingly, even after the antibiotic threat was lifted, both strains retained their plasmids at significantly higher levels than they had before the antibiotic exposure.
“When the plasmids could move between cells, greater stability gives a greater incidence of multi-drug resistance because a cell receiving one plasmid is more likely to already have a different one. In this way, we show that different populations with more stable plasmid-resistance systems are 'primed' to exhibit MDR when they co-mingle," Kerr told Laboratory Equipment.
Additional experiments by the researchers showed that antibiotic exposure increased the occurrence of MDR Klebsiella. According to the paper, when the researchers grew antibiotic-naive Klebsiella and E. coli plasmid-bearing strains together, a small fraction of Klebsiella became MDR by retaining their chloramphenicol-resistance plasmid and acquiring tetracycline-resistance plasmids from E. coli. But when the researchers repeated the experiment using antibiotic-exposed bacteria, they found roughly 1,000 times more MDR Klebsiella. This indicates that prior prolonged exposure to just one antibiotic—in this case chloramphenicol—exponentially increased the likelihood that chloramphenicol-resistant Klebsiella would acquire the tetracycline-resistance plasmid from E. coli and morph into an MDR bacteria.
"The ability of a strain to acquire and hold a plasmid is likely dependent on the plasmid and the strain of interest. There are strains of E. coli that can acquire readily certain plasmids and transfer them. However, why different species/strains receive/hold/transfer plasmids at different rates is an extremely interesting question—and one we want to explore in the future," said Kerr.
Future work will also examine whether evolutionary changes in bacteria affect the rate of plasmid transfer. Kerr said this kind of research requires new techniques to measure transfer rate, which the team is currently working on.
"We would also like to explore whether the origin of multi-drug resistant strains holding multiple plasmids sets the stage for multi-drug resistant plasmids, i.e., plasmids that have multiple resistance genes," he said.
Photo: Colorized scanning electron micrograph image of Klebsiella pneumoniae interacting with a human neutrophil, a type of white blood cell. Credit: National Institute of Allergy and Infectious Diseases