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Competition is a strong driving force of
evolution for organisms of all sizes: Those individuals best equipped to obtain
resources adapt and reproduce, while others may fall by the wayside. Many
organisms—mammals, birds, and insects, for instance—also form cooperative
social structures that allow resources to be defended and shared within a
population.
But surprisingly, even microbes, which are
thought to thrive only when able to win the battle for resources against those
nearest to them, have a somewhat sophisticated social structure that relies on
cooperation, according to Massachusetts Institute of Technology (MIT)
scientists. These researchers have recently found evidence that some ocean microbes
wield chemical weapons that are harmless to close relatives within their own
population, but deadly to outsiders.
The weapons are natural antibiotics
produced by a few individuals whose closest relatives carry genes that make
them resistant. The researchers believe that the few antibiotic producers are
acting as protectors of the many, using the antibiotics to defend the
population from competitors or to attack neighboring populations.
“We can’t know what the environmental
interactions really are, because microbes are too small for us to observe them
in action,” says Professor Martin Polz of MIT’s Department of Civil and
Environmental Engineering (CEE), lead investigator on a study appearing in Science. “But we think the antibiotics play a role
in fending off competitors. Of course, those competitors could also produce
antibiotics. It’s a potential arms race out there.”
A population of ocean microbes is defined
by genetic likeness and shared ecological activities, such as their preferred
microhabitat—say, free-floating or attached to algae—or their ability to
harvest a particular substance. Because close relatives within populations have
very similar if not identical resource requirements, they must by necessity also
be strong competitors with one another.
This makes cooperation involving
antibiotics doubly surprising, because the ability to produce antibiotics is a
classic example of a “selfish” gene that ought to increase the fitness—or
reproductive rate—of the individual carrying the gene. In a strictly
competitive environment, the microbe would use this advantage against its
closest relatives. But now it looks as if this competition is modulated by
social interactions where antibiotics produced by a few individuals act as “public goods”: items that benefit the group, rather than just the individual.
This differentiation of populations into
individuals that produce antibiotics and those that are resistant is one of the
first demonstrations that microbial populations engage in a division of labor
by social role. This observation also provides an explanation for why so many
genes are patchily distributed across genomes of closely related microbes. At
least some of these genes may be responsible for creating tightly knit social units
of bacteria in the wild.
“It’s easy to imagine bacteria in the
environment as selfish creatures capable only of reproducing as fast as
conditions allow, without any social organization,” says Otto Cordero, a CEE
postdoctoral research who is a first author on the Science paper. “But that is the
mind-blowing part: Bacterial wars are organized along the lines of populations,
which are groups of closely related individuals with similar ecological
activities.”
The study also uncovers an untapped source
of antibiotics that could have the potential to aid in the fight against human
bacterial pathogens, which are rapidly developing resistance to the few
antibiotics in use—nearly all of which are produced by soil-living bacteria.
“This paper [shows] that bacteria work
together in complex relationships that have largely been underappreciated by
the research community as a whole,” says Gerry Wright, a professor of
biochemistry and biomedical sciences and director of the Michael G. DeGroote
Institute for Infectious Disease Research at McMaster University. He adds, “The
impact on our understanding of resistance is critical. … This work is really
important in showing that we can, in fact, study these big questions in
populations of natural bacteria, and we can learn something important about how
we use antibiotics and avoid resistance in the clinic.”
To obtain these findings, the researchers
tested about 35,000 interactions among pairs of 185 strains of Vibrionaceae bacteria populations taken
from the ocean. They found that 44% of the strains were able to inhibit the
growth of at least one other strain and 86% were inhibited by at least one
other strain. They then used genomic analysis to determine genetic kinship.
