What Doesn’t Kill Microbes, Makes Them Stronger

If you are taking antibiotics, your doctor will admonish you not to skip any pills and to continue the treatment even after you start to feel better. That’s because failure to kill the bugs making you sick can cause some of them to become resistant to the antibiotics. Now, a new study explains how nonlethal antibiotic concentrations can lead to resistance. The drugs trigger the release of so-called reactive oxygen species (ROS) inside bacteria, which in turn cause mutations in the bugs’ DNA–including some that happen to cause resistance.

Traditionally, the development of antibiotic resistance–a big and growing problem in medicine–has been seen as a passive phenomenon. Haphazard mutations occur in bacterial genomes, and bacteria randomly swap genetic elements. Every now and then, a mutation or a bit of newly acquired DNA enables the microbes to detoxify antibiotics, pump them out of the cells, or render them harmless in another way. When these microbes are exposed to antibiotics, natural selection will allow them to outcompete the ones that aren’t resistant.

But in the past 6 years, a different view has emerged, says microbiologist Jesús Blázquez of the Spanish National Research Council in Madrid. Researchers have discovered that mutation rates in bacteria sometimes go up in response to stress, in some cases promoting resistance. And studies by Blázquez and others have shown that the antibiotics themselves can cause this phenomenon, called hypermutability.

The new study, led by systems biologist James Collins of Harvard University explains how this is possible. A few years ago, Collins’s group discovered that antibiotics can trigger the production of ROS, also known as free radicals, which can cause mutations in DNA. At high levels, the group discovered at the time, these mutations helped kill the microbes. But what about nonlethal doses of antibiotics, the researchers wondered. Could they, through the release of ROS, trigger the very mutations that make bacteria resistant?

To find out, the group treated Escherichia coli bacteria with low levels of the antibiotics norfloxacin, ampicillin, and kanamycin. The drugs increased levels of ROS, the team reports today in Molecular Cell. Using a simple procedure to estimate the number of mutations occurring in a cell culture, the team found that higher ROS levels led to higher mutation rates in the bacterial genomes–up to an eightfold rise in the case of norfloxacin. Next, they showed that low-level treatments did indeed trigger resistance–in many cases, not just against the drug itself, but to a whole series of other antibiotics as well.

The probable explanation, says Collins, is that antibiotics create a “whole zoo of mutants” in a bacterial population–including some that happen to be resistant to one or more drugs. The findings could have a practical upshot, Collins says. For instance, if researchers could find molecules that prevent hypermutability, they could be combined with antibiotics to prevent or delay resistance.

The paper provides more evidence that antibiotics aren’t just selecting certain mutations, but causing them, says molecular geneticist Susan Rosenberg of Baylor College of Medicine in Houston, Texas. “And they have shown that the mechanism involved is the release of reactive oxygen species,” she says. The paper also reinforces just how versatile microbes are, Blázquez adds. “Again, it seems that bacteria use adversity as a stimulus to adapt to almost everything,” he says.