Bacterial clusters living in the lungs of a cystic fibrosis patient are highly resistant to killing by antibiotics. Image: Singh lab
infections, even those caused by antibiotic-sensitive bacteria, resist
treatment. This paradox has vexed physicians for decades, and makes some
infections impossible to cure.
key cause of this resistance is that bacteria become starved for
nutrients during infection. Starved bacteria resist killing by nearly
every type of antibiotic, even ones they have never been exposed to
What produces starvation-induced antibiotic resistance, and how can it be overcome? In a paper appearing this week in Science, researchers report some surprising answers.
become starved when they exhaust nutrient supplies in the body, or if
they live clustered together in groups know as biofilms,” said the lead
author of the paper, Dr. Dao Nguyen, an assistant professor of medicine
at McGill University.
are clusters of bacteria encased in a slimy coating, and can be found
both in the natural environment as well as in human tissues where they
cause disease. For example, biofilm bacteria grow in the scabs of
chronic wounds, and the lungs of patients with cystic fibrosis. Bacteria
in biofilms tolerate high levels of antibiotics without being killed.
chief cause of the resistance of biofilms is that bacteria on the
outside of the clusters have the first shot at the nutrients that
diffuse in,” said Dr. Pradeep Singh, associate professor of medicine and
microbiology at the University of Washington in Seattle, the senior
author of the study. “This produces starvation of the bacteria inside
clusters, and severe resistance to killing.”
was previously thought to produce resistance because most antibiotics
target cellular functions needed for growth. When starved cells stop
growing, these targets are no longer active. This effect could reduce
the effectiveness of many drugs.
this idea is appealing, it presents a major dilemma,” Nguyen noted.
“Sensitizing starved bacteria to antibiotics could require stimulating
their growth, and this could be dangerous during human infections.”
Nguyen and Singh explored an alternative mechanism.
have long known that when bacteria sense that their nutrient supply is
running low, they issue a chemical alarm signal. The alarm tells the
bacteria to adjust their metabolism to prepare for starvation. Could
this alarm also turn on functions that produce antibiotic resistance?
test this idea, the team engineered bacteria in which the starvation
alarm was inactivated, and then measured antibiotic resistance in
experimental conditions in which bacteria were starved. To their
amazement, bacteria unable to sense starvation were thousands of times
more sensitive to killing than those that could, even though starvation
arrested growth and the activity of antibiotic targets.
experiment was a turning point,” Singh said. “It told us that the
resistance of starved bacteria was an active response that could be
blocked. It also indicated that starvation-induced protection only
occurred if bacteria were aware that nutrients were running low.”
the exciting result in hand, the researchers turned to two key
questions. First does the starvation alarm produce resistance during
actual infections? To test this the team examined naturally starved
bacteria, biofilms, isolates taken from patients, and bacterial
infections in mice. Sure enough, in all cases the bacteria unable to
sense starvation were far easier to kill.
second question was about the mechanism of the effect. How does
starvation sensing produce such profound antibiotic resistance?
Again, the results were surprising.
of well-described resistance mechanisms, like pumps that expel
antibiotics from bacterial cells, the researchers found that the
bacteria’s protective mechanism defended them against toxic forms of
oxygen, called radicals. This mechanism jives with new findings showing
that antibiotics kill by generating these toxic radicals.
The findings suggest new approaches to improve treatment for a wide range of infections.
new antibiotics has been challenging,” Nguyen said. “One way to improve
infection treatment is to make the drugs we already have work better.
Our experiments suggest that antibiotic efficacy could be increased by
disrupting key bacterial functions that have no obvious connection to
work also highlights the critical advantage of being able to sense
environmental conditions, even for single-celled organisms like
bacteria. Cells unaware of their starvation were not protected, even
though they ran out of nutrients and stopped growth. This proves again
that, even for bacteria, “what you don’t know can hurt you.”