When competition is intense, viruses spill over into new hosts

By Ed Yong | October 16, 2012 7:00 pm

When you think about viruses, you might wonder how they infect, how they spread, and how they kill. These questions are of natural interest—you, after all, could play host to a grand variety of lethal viruses. But do remember: it’s not all about you.

A virus’ world contains not just potential hosts, but other viruses. It has competition. This simple fact is often ignored but it has profound implications. In a new study, Lisa Bono from the University of North Carolina has shown that competition between viruses can drive them to spill over into new hosts, imperilling creatures that they never used to infect.

Rather than focusing on the viruses we know and dread, Bono worked with phi6, a virus that infects and kills bacteria. This bacteriophage (hereafter just ‘phage’) looks like a lunar lander, that docks onto the surface of bacteria with spindly ‘legs’ and injects its genetic material through a syringe-like tube. Bono worked with a phage strain that infects Pseudomonas syringae, a bacterium that causes disease in plants. It favours this species above all others.

Bono added her phages to tubes containing either its normal host P.syringae, or a close relative—P.atrofaciens or P.glycinea. She delivered the viruses at two different concentrations, one a hundred times greater than the other, to simulate either weak or strong competition between them.

After several rounds of infection, Bono found that the phages that faced stiffer competition for their normal host bacterium , also quickly evolved to infect an alternative one. They changed from specialists to generalists. And they didn’t lose much in this transformation—they could still infect P.syringae almost as effectively as their specialised cousins, and often out-competed and replaced them.

These ideas aren’t new. Several scientists had predicted that species could evolve new behaviours, or even split into two, because of competition. If everyone’s fighting, some individuals would fare better by exploiting a new resource, even if it’s not as good as their usual fare. Put it another way: if everyone is competing over chocolate, you’ll eat well if you switch to apples, even if that’s the less enticing option

It’s a reasonable idea, but experimental evidence is rare. Here’s one of the few examples: in 2001, Daniel Wolnick showed that fruit flies adapted more quickly to food laced with toxic cadmium if they faced greater competition for food. Because of their rivalry, their tastes broadened. Bono’s study is another.

The broader point – that competition between infections can make things worse for hosts – applies beyond viruses. It also has direct relevance to our own health.  Here’s one example: Escherichia coli, a typically harmless gut bacterium, can cause disease in mice after it’s exposed to the threat of amoebas. That’s because the strains that can survive being eaten by the amoebas, are the same ones that can reproduce inside our own cells and cause disease.

And another one: a normally harmless nose bacterium Streptococcus pneumonia becomes infectious when it battles against another species Haemophilius influenzae. Streptococcus surrounds itself with a thicker coat to ward off the white blood cells summoned by its competitor. In doing so, it become better armoured against our own immune system, and starts to cause disease.

It’s germ warfare, and even though we’re not fighting, we still stand to lose.

Reference: Bono, Gensel, Pfennig & Burch. 2012. Competition and the origins of novelty: experimental evolution of niche-width expansion in a virus. Biology Letters http://dx.doi.org/10.1098/rsbl.2012.0616

CATEGORIZED UNDER: Evolution, Medicine & health, Viruses

Comments (5)

  1. W. Benson

    Other studies showing intraspecific competition favors the evolution of reduced specialization (wider niches):

    Daniel I. Bolnick. 2001. Intraspecific competition favours niche width expansion in Drosophila melanogaster. Nature, 410: 463-466. ´[paywall]

    Richard Svanbäck + Daniel I. Bolnick. 2007. Intraspecific competition drives increased resource use diversity within a natural [stickleback] population. Proceedings of the Royal Society, B, 274: 839-844. [free pdf where I live]

  2. Totto

    “The broader point – that competition between infections can make things worse for hosts – applies beyond viruses. It also has direct relevance to our own health.”

    This is important. And to think about how to experimentally provide evidence of the nature. Maybe it will lead to understanding of the virulence of the agent itself. Interesting read :)

  3. David E

    This is totally unconvincing. It is not clear that competition had anything to do with the result. Some individuals of ph16 may have the ability to infect P.glycinea. In a 100-fold sample they should certainly be more likely to occur. Given only P.glycinea as a substrate, they will flourish while the typical individuals die out. In the meagre sample, perhaps none or extremely few of these types occur.

  4. Field

    This is confusing because it implies viruses are naturally happy with a certain level of food, but the reality of what we know about them (or that I know) is closer to bacteria. They are opportunistic in nature, always seeking to eat and multiply. That is what they do… just in different ways.

    Could it be these more robust versions of viruses don’t naturally reproduce as fast, so we don’t typically see them as much? But if you stress the immune system you allow them to propagate consistently for a longer period of time?

    When I read this, my thought was ‘cure for HIV?’ I was being optimistic thinking about the possibility of displacement. But actually the possibility for something more like super-HIV could occur. I guess testing on how HIV responds to competition, in evolution, could be interesting. Is it possible for a virus to evolve so well that by creating such a strong outer-layer it could actually slow down how fast it replicates, and perhaps squelch the ability to interact with a lot of the regular food buffet it is use too? See if you introduced competition that had to be fed with introduced food, but it forced/allowed changes in the HIV, you could selectively stop the quick changes by not feeding the introduced virus as the most opportune time.

    What if the person who was cured with the bone marrow transplant wasn’t cured because of the re-energized immune system with fresh not-so-taxed marrow, but rather because of the gap in immunity that allowed evolution of the HIV to turn into something that was giving heritable qualities associated not with speed and evasion but something else?

    I suppose the idea might just be a way to kill the host quicker, but I’ll stick with optimism.

  5. A. Peon

    This writeup seems awfully anthropomorphic (evolution ‘wants’ to happen! competition makes things ‘better’!). Viruses, including bacteriophages, are basically chemicals (and so are people, but the complexity on the viral scale is much reduced). If the probability of a ‘favorable’ mutation is fixed per unit of viruses, should it really be ‘surprising’ that adding more viruses increases the speed with which that mutation appears and is replicated enough to detect?

    This is useful science, and it’d be particularly interesting to see this uncovering latent mechanisms that actually exist in a ‘stock’ genome (such as the human pathogen examples given), but from the description I’m not seeing how this particular scenario demonstrates anything other than that ‘statistics happens’ – you have some available hosts the standard virus didn’t come ‘built for’, and the chance of a mutation that permits infection increases as you have more viruses so more possibilities for mutation.

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