Bacterial Warriors

Bacterial Warriors Scanning electron micrograph (SEM) of Vibrio vulnificus bacteria.
Photo Credit: CDC.gov

Populations of ocean bacteria have a few designated fighters, which release antibiotics that don't harm their own community.


Microbial militaries.  I’m Bob Hirshon and this is Science Update.

Some ocean bacteria rely on a few good microbes to wage war and defend the community.  MIT evolutionary ecologist Martin Polz and his colleagues reported this in the journal Science. Polz says that within colonies of saltwater bacteria called Vibrio, only a handful of individuals produce antibiotics. Those antibiotics kill bacteria from competing populations, but their own communities are immune to them.

They’re specialized defenders, if you will, within the population, that take on a special role that benefits the group.  What we talk about then, is that an antibiotic that’s produced within the population is, in a way, a public good.

That kind of a social structure wasn’t expected in these single-celled organisms.  The question now is whether different bacteria in the community perform other specialized jobs.  I’m Bob Hirshon for AAAS, the Science Society.

Making Sense of the Research

In modern human societies, people play specialized roles in order to make society operate more efficiently.  Farmers grow food; police fight crime; construction workers build roads, and so on.  Militaries are also one of those specialized groups. Nations train and maintain relatively small groups of people to fight wars and defend the country, rather than relying on all citizens to rise up in their own defense.

Many animals, even insects, assign individuals to specialized social roles as well.  Bees and ants are among the best-known examples.  In those cases, even breeding is a job – only a minority of individuals can biologically reproduce.  What's surprising here, though, is to find a kind of division of labor in single-celled bacteria.

The sea bacteria Polz' team studied belong to a genus called Vibrio.  Vibrio live in populations that may include several different species, which can freely exchange genetic material.  One type of Vibrios bacteria causes cholera, a water-borne bacterial infection that kills millions of people every year, mainly in developing countries.  So understanding their behavior is of more than just academic interest.

The bacteria can use antibiotics as weapons against competing populations.  Polz' team found that within a given population, only a few individuals are actually capable of producing antibiotics.  But almost all of the bacteria in their own population were immune to them. 

The latter part isn't surprising, since you would expect that any non-immune bacteria within a population would quickly die off.  What is surprising is that the job of making the antibiotics is reserved for just a small subset of the population.  You might expect that if defensive antibiotics are useful, all the bacteria would make them.

The fact that they don't suggests that there's some kind of cost to making the antibiotics, according to Polz.  For example, it may take a considerable amount of energy, which could otherwise go to essential biological functions.  If that's the case, Polz says it's likely that other bacteria in a population take on different specialized jobs, which come with different costs.  It may be that having each bacteria perform one job is more efficient than having every bacteria capable of everything.  If so, evolutionary pressures would favor communities that can work as a team.

Now try and answer these questions:

  1. What defensive substance to the sea bacteria produce?
  2. What's significant about the fact that only certain individuals can make it?
  3. Why do the researchers think other individual bacteria may perform other jobs?

You may want to check out these related resources:

For more about surprisingly complex interactions in seemingly unlikely organisms, see the Science Updates Plant Kin HelpSpider Web Sites, and Worm Language.

Going Further

For Educators

For more about surprisingly complex interactions in seemingly unlikely organisms, see the Science Update lessons Plant Kin HelpSpider Web Sites, and Worm Language.

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