A new study has discovered that for, some pathogens, the rules of evolution were made to be broken. In most living things evolution and adaptation are driven by vertical gene transfer (VGT) which sees the downward transfer of genetic material from a parent to offspring. Now, however, scientists are recognizing an alternative form of DNA transfer that sees genetic material passed horizontally between bacteria, bending the rules of evolution as beneficial traits emerge among populations that haven’t been exposed to the evolutionary pressure that forged a specific adaptation in the first place.
The genes that enable bacteria to survive antibiotics are commonly found in hospitals, sewers, and farms because these places are exposed to antibiotics as the result of human activity, be it treating patients, water, or animals. The slightly perplexing incidence of these genes is when they’re found in places not exposed to antibiotics in this way, such as forests and estuaries. Even though the bacteria that are found here aren’t pitted against antibiotics, triggering the adaptation of resistance, bacteria are still found containing these resistant genes. The concerning emergence of these genes has seen antibiotic resistance more than double in the last 20 years.
Researchers wanted to understand how these resistant genes were spreading in the environment by looking at antibiotic-sensitive bacteria in a lab. They kept it in a growth media, which didn’t contain antibiotics but allowed it to receive HGT from antibiotic-resistant bacteria. Using whole-genome sequencing they were able to confirm that the genes for antibiotic resistance were spreading in the populations, even without selection. These HGT populations were then pitted against high concentrations of antibiotics and were able to survive while control populations didn’t.
The discovery reveals why antibiotic resistance evolves so quickly in hospitals, and how antibiotic resistance genes can emerge in a population, even when there is no antibiotic selection pressure.
“Bacteria (and their DNA) are constantly transported across great distances by wind, water, animals, and humans,” study author Dr Mike McDonald told IFLScience. “When bacteria, or the DNA from bacteria, are moved around this provides an opportunity for horizontal gene transfer to happen."
It was previously thought that adaptations could only spread through populations that had been exposed to a selection pressure, but this research shows that, in antibiotic resistance at least, genes don’t need to confer a benefit in order to spread in the population.
"What we found in our laboratory experiment is that antibiotic genes can establish in growth conditions even without antibiotics and even when they do not confer a benefit, as long as there is enough horizontal gene transfer and the deleterious cost of the gene is not too high," Dr McDonald said.
“This research tells us that the use of antibiotics in one location, for example a farm, could lead to antibiotic resistance to spread to another location, where the genes could recombine into new, dangerous combinations. Our results emphasize the importance of careful stewardship of antibiotics, and surveillance to monitor the prevalence of antibiotic resistance genes.”