Shigella flexneri LuxS quorum-sensing system modulates virB expression butis not essential for virulence

Quorum-sensing systems regulate the expression of virulence factors in a wi de variety of plant and animal pathogens, including members of the Enteroba cteriaceae. Studies of Shigella virulence gene expression have demonstrated that maximal expression of genes encoding the type III secretion system an d its substrates and maximal activity of this virulence organelle occur at high cell density. In these studies, we demonstrate that the expression of ipa, mxi, and spa invasion operons is maximal in stationary-phase bacteria and that conditioned media derived from stationary-phase cultures enhance t he expression of these loci. In contrast, expression of virB, a transcripti on factor essential for the expression of invasion loci, peaks in late log phase; accordingly, virB expression is enhanced by a signal(s) present in c onditioned media derived from late-log-phase cultures. Autoinducer 2 (AI-2) , a quorum signaling molecule active in late log phase, was synthesized by Shigella species and enteroinvasive Escherichia coli and shown to be respon sible for the observed peak of virB expression. However, AI-2 does not infl uence invasion operon expression and is not required for Shigella virulence , as mutants deficient in AI-2 synthesis are fully virulent. The implicatio ns of these findings with regard to both virB and invasion operon expressio n and the evolution of circuitries governing virulence gene expression are discussed.