Controlling a Necessary Evil: Quorum Sensing Attenuates Virulence in Sodalis praecaptivus.

Sodalis praecaptivus is a close relative and putative environmental progenitor of the widely distributed, insect-associated, Sodalis-allied symbionts. Mutant strains of S. praecaptivus, lacking critical components of a quorum sensing (QS) apparatus acquire a rapid and potent killing phenotype following microinjection into an insect host. Transcriptomic and genetic analyses indicate that insect killing occurs as a consequence of virulence factors, including insecticidal toxins and enzymes that degrade the insect integument that are normally repressed by QS at high infection densities. This unorthodox method of regulation ensures that virulence factors are only utilized to initiate the insect-bacterial association. Once bacteria reach a sufficient infection density in host tissues, the QS circuit represses the expression of these harmful genes, facilitating a long-lasting and benign association. We discuss the implications of the functionality of this QS system in the context of the establishment and evolution of mutualistic relationships involving these bacteria. mRNA profiles of exponential growing bacterial culutres in rich media for wild type (WT), ∆ypeI, ∆ypeR, ∆yenR were generated by deep sequencing, in triplicate, using Illumina TruSeq method on a HiSeq instrument.