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Viruses evolve to prevent bacterial hosts from committing suicide

9 November 2012

University of Cambridge researchers have discovered an extraordinary way that bacterial parasites prevent their hosts from killing themselves to protect the wider colony.

Researchers funded by BBSRC discovered that a strain of the potato soft rot and blackleg bacterium Pectobacterium atrosepticum (AKA Erwinia) had evolved to commit suicide in the presence of certain viral parasites, known as bacteriophages, to limit the spread of viral infection in the wider bacterial population.

Some mutant bacteriophages have evolved to stop their hosts from committing suicide. iStockphoto, Thinkstock 2012

Some mutant bacteriophages have evolved to stop their hosts from committing suicide.
Image: iStockphoto, Thinkstock 2012

The bacterial cells in a population that commit "suicide" by dying prematurely can be viewed as acting "altruistically", giving up their lives to prevent viral replication in siblings in the rest of the culture, in a process called abortive infection.

The paper, Viral Evasion of a Bacterial Suicide System by RNA-Based Molecular Mimicry Enables Infectious Altruism, published in PLoS Genetics on October 18 2012, also identified how the evolutionary arms race had produced low numbers of bacteriophage mutants that could suppress bacterial suicide in infected cells. These rare mutants (from a new transducing bacteriophage of the Myoviridae family) had evolved to produce an RNA antitoxin similar to that normally manufactured by the bacteria to suppress the lethality of the endogenous toxin, thus evading the suicide defence mechanism. By producing the mimic antitoxin, the virus could continue replicating without becoming a victim of the host's defensive system.

The mutant bacteriophage was also able to transfer DNA encoding the defence system to a new bacterial host. In doing so it may have indirectly created populations of host cells inside which it could successfully replicate while potentially providing the new host with better protection from competing viral predators.

Professor George Salmond, deputy head at Cambridge University's Department of Biochemistry, said: "This work highlights the incredibly dynamic world of adaptive co-evolution in bacteria and their viruses. The emergence of an RNA-based molecular mimicry in the virus to suppress bacterial suicide is an exciting observation.

"Furthermore, multiple alternative and novel routes, through which different bacteriophages may evolve to evade abortive infection, remain to be discovered. Because the bacteriophage investigated can pick up DNA from one bacterium and transfer it to a new host, this meant that escape mutants might be able to transfer the abortive infection system to other hosts - and that was confirmed. In effect, this could be viewed as an example of 'infectious altruism' - with a virus acting as a vector to transmit an anti-viral defence system between bacteria.

"Hypotheses about the molecular evolution of the "altruistic" trait, and possible adaptive impacts and fitness consequences (for both virus and bacterial host) can now be formally tested."

ENDS

Notes to editors

Paper: 'Viral Evasion of a Bacterial Suicide System by RNA-Based Molecular Mimicry Enables Infectious Altruism', published in PLoS Genetics on October 18 2012: www.plosgenetics.org/article/info%3Adoi%2F10.1371%2Fjournal.pgen.1003023

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