Abstract: Ralstonia solanacearum is a soil- and water-borne bacterium that attacks plant vascular systems. It only produces its costly virulence factors when it has reached high populations inside a host. To identify traits that this pathogen needs in other life stages, we studied a mutant locked into its low cell density mode.
This mutant (PhcA) cannot sense its own population density. It grew faster and used dozens of nutrients not available to the wild-type bacterium. The mutant also attached to itself and to plant root surfaces much better, yet it failed inside plants because it formed dense mats that kept it from spreading. Thus, PhcA is a switch that helps R. solanacearum succeed over its complex life cycle by mediating two strategic trade-offs: between growth and virulence, and between attachment and spread.