The way the bacteria's evolution played out surprised the
researchers in a couple of ways.
Crafty bacteria that mutate and evade the forces of current
drugs are a real concern to the global community, with a recent
report predicting that these superbugs could kill 10 million
people a year by 2050, an average of one death every three
seconds. Scientists have built a new tool to study how the
microscopic killers operate, taking the form of giant Petri dish
where they can be seen evolving resistance to rising
concentrations of antibiotics in a relatively short space of
time.
Much of the time, scientists study the evolution of microbes
in the lab by pitting them against drug mixtures in relatively
small spaces, like on tiny plates. Researchers at the Harvard
School of Medicine (HMS) together with colleagues at the
Technion-Israel Institute of Technology sought to create an
environment that would more accurately mimic real-world
conditions, allowing factors like space, size and geography to
come into play.
"We know quite a bit about the internal defense mechanisms
bacteria use to evade antibiotics but we don't really know much
about their physical movements across space as they adapt to
survive in different environments," says study first author and
research fellow in systems biology at HMS, Michael Baym.
So the team built a huge Petri dish that they could use to
observe bacterial behavior as the microbes encountered different
concentrations of antibiotic drugs. Measuring 2 x 4 ft (0.6 x
1.2 m), the dish is split into nine different reservoirs running
the length of the dish. All are filled with agar, a jelly-like
substance taken from seaweed used in science experiments to
nourish growing organisms, but each has differing concentrations
of antibiotic drugs.
The sections at each of end contained no drugs at all, while
the second from the end contained just above the minimum
required to kill the bacteria. These dosages increased ten-fold
in each section toward the center, with dosages of 10 times, 100
times and then 1,000 times more than needed to kill the
bacteria.
By mounting a camera on the ceiling, the team created a
time-lapse video (see below) that showcases evolution in all its
glory. They introduced E. coli to the end sections and
watched as the bacteria adapted, survived and thrived on each
level until conquering the highly lethal dosage in the middle,
standing triumphantly as superbugs capable of enduring a drug
dosage 1,000 times stronger than that which killed their
ancestors just 10 days earlier. When the scientists switched
antibiotics, from trimethoprim to ciprofloxacin, the bacteria
exhibited a 100,000-fold increase in drug resistance.
Dubbed the Microbial Evolution and Growth Arena (MEGA), the
giant plate was designed as an educational tool so that students
could see evolution playing out before their eyes, but the
researchers found that it has revealed some useful insights
about drug-resistant bacteria, too.
As the bacteria spread into new concentration levels, only a
small portion were able to adapt and survive. Mutations through
the generations would eventually give rise to drug-resistant
microbes whose descendants then moved into new frontiers, and
so-forth until they reached the center. But the way this played
out surprised the researchers in a couple of ways.
They found that the initial mutations applied the brakes
somewhat, with growth slowing as the bacteria adapted to each
higher dose of antibiotics, before returning to normal growth
rates once they had become fully resistant. Their observations
also challenge the traditional assumption that the most
resistant mutants are those responsible for driving evolution.
By studying mutants both at and behind the front lines, the team
found that this was not always the case.
"What we saw suggests that evolution is not always led by the
most resistant mutants," says Baym. "Sometimes it favors the
first to get there. The strongest mutants are, in fact, often
moving behind more vulnerable strains. Who gets there first may
be predicated on proximity rather than mutation strength."
While it doesn't perfectly replicate how bacteria behave in
real-world environments, such as hospitals, the researchers say
the MEGA-plate is a more accurate representation than typical
lab cultures.
"Our MEGA-plate takes complex, often obscure, concepts in
evolution, such as mutation selection, lineages, parallel
evolution and clonal interference, and provides a visual
seeing-is-believing demonstration of these otherwise vague
ideas," says senior study investigator, Roy Kishony. "It's also
a powerful illustration of how easy it is for bacteria to become
resistant to antibiotics."
You can watch the bacteria take over the MEGA-plate in the
video below, while the team's research was published in the
journal
Science.
Source:
Harvard School of Medicine