There is a debate about whether antibiotics influence the rate at which bacteria acquire drug resistance, with some researchers showing that exposure to antibiotics increases the spread of antibiotic resistance through a bacterial population. But work from scientists at Duke University suggests that genes that confer antibiotic resistance are not shared more frequently when antibiotics are present. Their work has also shown that at least 25 percent of antibiotic-resistant bacterial pathogens can spread their resistance genes directly to other bacteria.
Gif courtesy of Duke University
This work, which was reported in Science Advances, used a rapid technique to measure how fast bacteria shared circular, mobile pieces of bacterial DNA called plasmids, which often carry resistance genes. Using a high-throughput, automated process may help show which variables influence the rate at which plasmids are shared, and may help researchers stop resistance from spreading.
"Our previous research showed that antibiotics do not affect the rate at which bacteria spread their resistance directly to their community in laboratory strains of E.coli," said Lingchong You, professor of biomedical engineering at Duke. "But we wanted to see if this is also true for clinical strains of pathogens that are actually out there in the world."
Bacteria can easily share plasmids with one another in a process called bacterial conjugation (explained in the video below). The plasmids don’t all carry the same gene; different ones carry different genes, some of which enable bacteria to avoid the impact of antibiotics. Antibiotics often come from natural sources like fungus, and resistance genes will probably always be floating around in the microbial community. But there may be ways to discourage or prevent bacteria from swapping those genes.
"So the real problem is the resistance making its way into pathogens that harm humans," said the first study author Jonathan Bethke, a graduate student in the You laboratory. "We're looking to gain a good understanding of what factors affect their rate of conjugation, because if you can slow that process down enough, the plasmids carrying the genes for resistance can fall out of a population."
Plasmid conjugation is usually measured by observing hudreds of bacterial colonies that take 16 hours to replicate. This laborious process was replaced by a method created by Bethke and Allison Lopatkin, now an assistant professor at Barnard College, which delivers results in five hours. Two strains of bacteria are combined, one carrying a mobile antibiotic resistance element on a plasmid (green in the video above), the other carrying resistance to a different antibiotic that cannot be shared (red in the video above). The strains live together to allow conjugation to occur, and then the microbes are moved to an environment containing both antibiotics. The bacteria that carried non-sharable resistance, which also acquired a plasmid through conjugation, can now grow while the other microbes cannot (the red bacteria take over the culture). After a time, the researchers can determine how many microbes there were with dual resistance.
"This method opens up the ability to test many more drugs or environmental factors to see how they influence the rate of plasmid conjugation," said Bethke. "It will also allow us to determine if there's some sort of genetic determinant that is playing a greater role in terms of the transfer rate."
The researchers also obtained samples from patients that had bacterial infections, all of which were resistant to a common kind of antibiotic called a beta-lactamase. The presence of beta-lactamase did not make bacteria share resistance genes faster, except in a single case. About 25 percent of the strains could however, share their resistance genes rapidly.
"We were surprised to discover that it was that high," said You. "And of course antibiotics do promote the spread of resistance, but our study indicates that it is primarily through selective population dynamics rather than through an increased rate of plasmid conjugation."