Biofilms as “Selfish Herds”

Plant roots are often covered with bacterial biofilms and it’s sometimes assumed that these bacteria must be working together for the good of the plant. Although the bacteria may often be more beneficial than harmful, any benefits to the plant are usually side-effects of activities each bacterial strain does to benefit itself. For example, toxins the bacteria make to kill competitors may, as a side-effect, kill fungi that might otherwise harm the plant. Because evolution of toxin production doesn’t depend on benefits to the plant, however, the toxins themselves sometimes damage roots.

Nuno Oliveria and colleagues just published a paper in PLoS Biology confirming earlier observations that low doses of antibiotics made by bacteria can stimulate biofilm production. This result has sometimes been seen as evidence that antibiotics are “signals” for cooperation among bacterial strains, rather than weapons. But Olivera et al. found that

“one strain normally kills the other off… biofilm formation is actually a response to the damage of antibiotic warfare.”

Will Ratcliff and I made a similar suggestion in 2011, in a paper cited by Olivera et al. While we were waiting for Science to decide whether to accept our paper, I posted the following on my (now extinct) blog, This Week in Evolution:


“Scientists once thought that wolves chase deer and may even try to eat them, that sharks attack other fish, and that cold weather can kill penguins. But recent research has shown that these so-called threats are actually beneficial, because they encourage togetherness.”

OK, I made the above “quotation” up, but the logic is the same as in a recent story in Science about antibiotics:

“there’s scant evidence that bacteria or fungi deploy antibiotics to kill or ward off other microbes… These molecules, they assert, may be less weapons for competition or combat than tools of communication… When certain bacteria are exposed to nystatin, the microbes form slimelike communities known as biofilms… this may be just one of the molecule’s natural roles.”

For a bacterial cell, there are both disadvantages and advantages to crowding together in a biofilm. There may be more competition for available nutrients in a biofilm, but there may be more nutrients to compete for. One reason is that an individual bacterium may not be able to excrete enough enzymes to release nutrients from a solid surface; a bunch of bacteria growing in a biofilm may do better.

But why use antibiotics as signals to form biofilms, when bacteria can produce and detect plenty of nontoxic signals? See previous posts on “quorum sensing“, but also this discussion.

The most logical reason to form a biofilm in the presence of an antibiotic is to escape from the antibiotic! I am reminded of this quotation about cattle in lion country:

” Yet although the ox has so little affection for, or individual interest
in, his fellows, he cannot endure even a momentary severance from his
herd. If he be separated from it by strategem or force, he exhibits every
sign of mental agony; he strives with all his might to get back again and
when he succeeds, he plunges into its middle, to bathe his whole body
with the comfort of closest companionship.”

The quotation is from Francis Galton, but I got it from Bill Hamilton’s 1971 paper, “Geometry for the selfish herd.” Other examples in this classic paper — “Evilutionary Biologist” John Dennehy has written a nice summary — include reindeer at the edge of a herd suffering much more attack from parasitic insects and gulls nesting at the edge of a colony suffering much more predation.

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