Antibiotic resistance could have a drastic impact on all of
our lives, but is invisible to the naked eye and impossible for most to
comprehend. So researchers at Harvard Medical School and the Technion-Israel
Institute of Technology decided to make it obvious: They designed a model to
show bacteria mutating to overcome drugs meant to stop and destroy them.
Along the way, their experiments, which have been described
and videotaped for the journal Science, revealed something even more profound.
Not only does the model vividly illustrate how antibiotic resistance happens,
it also demonstrates "survival of the fittest" and other Darwinian
concepts that have been often discussed but never once seen.
"When I saw those videos it kinda hit me viscerally --
I'm watching evolution, I don't have to think about it, there it is, I can see
it," said Sam P. Brown, an evolutionary biologist at Georgia Institute of
Technology, who was not involved in the experiments. He and Luke McNally of the
University of Edinburgh, co-wrote a commentary published with the research.
To observe the do-or-die encounter between bacteria and an
antibiotic drug, the research team constructed a two-foot by four-foot petri
dish -- dubbed the Microbial Evolution and Growth Arena or MEGA plate -- and
filled it with agar, a jellylike nourishment used in labs to feed growing
organisms. Next, they searched among bacteria for the right one to work with
and landed on E. coli.
"In order to grow bacteria on a petri dish of that
size, it needs to be able to swim, which is something E. coli can do but many
other model organisms cannot," said Dr. Michael Baym, first author of the
study and a postdoctoral fellow in microbial evolution at Harvard Medical
School. E. coli also possesses fundamental mechanisms in common with
infection-causing bacteria, explained Baym, and quite simply, "we knew how
to work with it."
Next, the researchers divided the MEGA plate into sections
and added increasing doses of an antibiotic, trimethoprim. This antibiotic was
chosen because it is well-known, explained Baym. "We have a lot of
experience understanding how resistance of trimethoprim evolves," said
Baym. "We were developing a new system to study evolution and so we wanted
to work with as many parts that we sort of understood as possible."
Preparing the MEGA plate for their experiments, the research
team left the outermost area clean of trimethoprim. In the area nearest this
outermost section, they added a single dose of the drug and then, as the
sections progressed to the center, the dosage kept increasing until it reached
1,000 times the initial dose.
Over two weeks, a ceiling-mounted camera snapped periodic
shots of what transpired in the MEGA plate below. Later, the researchers
spliced these shots into a time-lapsed videotape that showed how most of the
bacteria spread until they reached the antibiotic dose that was too strong for
them to continue growing or living.
However, at each dosage level, a small group of bacteria were
able to survive. Bacteria, like other living beings, evolve to adapt to changes
in their environment and one way they do this is through genetics. New genes
can arise through mutations and these then get passed down to subsequent
generations.
The MEGA plate revealed all this and more: Descendants of
the drug-resistant mutants instinctively migrated to new territories, the areas
of fresh agar nourishment and also higher antibiotic concentration. Once there,
multiple lineages of mutants competed for dominance within the same space. And
so, over a span of just days, the wily E. coli strains made their way from the
good life of easy nourishment in a drug-free outer layer through sections of
the MEGA plate containing increasingly higher doses of antibiotic.
The early low-resistance mutants soon gave rise to
moderately resistant mutants and ultimately these intermediate bacteria spawned
highly resistant strains able to overcome a dose of trimethoprim 1,000 times
more intense than the one that killed their ancestors.
"You can just see mutations and selections and
trade-offs happening right in front of you," said Baym.
Along with the usual package of genes inherited from their
forebears, bacteria also contain plasmids, small rings of additional DNA.
Scientists have known that plasmids enable wider and faster spread of
resistance among bacteria. Within the MEGA plate, the bacteria strains traded
plasmids containing genes essential to survival…
In this second experiment, the E. coli mutated to develop
100,000-fold resistance to an initial dose of cipro and, as the researchers
anticipated, the evolutionary process "looked quite different" from
that of trimethoprim, according to Baym. Still, the branches of a tree could be
traced along the ever-more-resistant bacteria.
In both cases, though, location of a particular bacterial
strain determined its success --- or failure --- in developing resistance. When
the researchers moved mutants trapped behind their parents to the so-called
"frontlines" of the culture, they were able to grow into new regions
where the parents could not. Survival may not be driven by the fittest mutants,
suggest Baym and his colleagues; what matters most for survival is a combination
of sufficient fitness and sufficient closeness to the advancing front.
Courtesy of a colleague
Baym M, Lieberman TD, Kelsic ED, Chait R, Gross R, Yelin I,
Kishony R.
Spatiotemporal microbial evolution on antibiotic landscapes.
Science. 2016 Sep
9;353(6304):1147-51.
Abstract
A key aspect of bacterial survival is the ability to evolve
while migrating across spatially varying environmental challenges. Laboratory
experiments, however, often study evolution in well-mixed systems. Here, we
introduce an experimental device, the microbial evolution and growth arena
(MEGA)-plate, in which bacteria spread and evolved on a large antibiotic
landscape (120 × 60 centimeters) that allowed visual observation of mutation
and selection in a migrating bacterial front. While resistance increased
consistently, multiple coexisting lineages diversified both phenotypically and
genotypically. Analyzing mutants at and behind the propagating front, we found
that evolution is not always led by the most resistant mutants; highly
resistant mutants may be trapped behind more sensitive lineages. The MEGA-plate
provides a versatile platform for studying microbial adaption and directly
visualizing evolutionary dynamics.
For those who can access the article on line, there are videos of this process.
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