Editors' ChoiceMicrobiology

Quorum sensing to repress virulence

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Sci. Signal.  16 May 2017:
Vol. 10, Issue 479, eaan6287
DOI: 10.1126/scisignal.aan6287

A quorum-sensing system of an insect endosymbiont enables the bacterium to establish persistent infection by inhibiting the expression of virulence genes.

Bacteria use quorum-sensing systems to coordinate group behaviors, such as the formation of biofilms and activation of virulence factors. In such systems, a secreted microbial molecule triggers group responses when it accumulates in the environment above a certain threshold. This ensures that there are sufficient numbers of bacteria to support the group behavior. There are many examples of pathogenic bacteria that produce virulence factors only when a quorum has been reached. Enomoto et al. identified a quorum-sensing system in the grain weevil symbiont Sodalis praecaptivus that represses rather than stimulates the production of virulence factors. S. praecaptivus mutants lacking components of this quorum-sensing system grew poorly in culture compared with wild-type cells. Whereas wild-type bacteria infected weevils without causing the animals to die, the S. praecaptivus quorum-sensing mutants killed the hosts. The authors identified genes that were transcriptionally regulated by the quorum-sensing system and found that those target genes that were repressed included those that encoded virulence factors, such as insecticidal toxins and proteases. Deletion of some of these virulence factors reduced the ability of S. praecaptivus quorum-sensing mutants to kill weevils, implying that failure to inhibit these factors would kill the host rather than promote chronic infection. However, deletion of a gene encoding an insecticidal toxin limited the ability of S. praecaptivus to infect weevils, indicating that at least some virulence factors are required to initially establish infection. The authors identified a homologous quorum-sensing system in S. glossinidius, a related species that is an endosymbiont of tsetse flies, suggesting that the repression of virulence factors by quorum sensing may be a shared feature of bacteria that have evolved to cause persistent infection instead of killing the host. Commentary from Rio considers the implications of these findings in the greater context of the evolution of mutualistic relationships between bacteria and their hosts.

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