Pseudomonas aeruginosa adaptation to survive in the host hinges on its ability to probe the environment and respond appropriately. Rapid adaptation is often mediated by two-component regulatory systems, such as the PhoP/PhoQ system that responds to Mg ion concentration. However, there is limited information about the role of PhoQ in P. aeruginosa bloodstream infections. We used a murine model of systemic infection to test the virulence of a PhoQ-deficient mutant. Mutation of PhoQ impaired the virulence and the ability to cause bacteremia of P. aeruginosa. In the presence of blood concentrations of Mg , a PhoQ mutant bound more C3 and was more susceptible to complement-mediated opsonophagocytosis than the parent strain, suggesting a direct effect of the Mg on the modulation of expression of a bacterial component controlled by the PhoP/PhoQ system. Ligand blot analysis, C3 binding experiments and opsonophagocytosis assays identified this component as the outer membrane protein OprH, expression of which impaired the virulence of P. aeruginosa in a murine model of systemic infection. We demonstrate that expression of PhoQ is essential to detect Mg and reduce the expression of OprH, a previously unrecognized C3 binding molecule that promotes the opsonophagocytosis of P. aeruginosa.

Surfactant protein A (SP-A) plays a critical role in the clearance of Pseudomonas aeruginosa from the lung. However, there is limited information about the interaction of this protein with P. aeruginosa isolates from individuals with cystic fibrosis (CF). We characterized the interplay between SP-A and a collection of isogenic sequential isolates from 7 patients with CF. We identified outer membrane protein OprH as a novel ligand for SP-A on P. aeruginosa The last-available (late) isolates from patients with CF bound significantly less SP-A than their respective first-available (early) isolates. This difference could be associated with a reduction in the expression of OprH. Binding of SP-A to OprH promoted phagocytic killing; thus, late CF isolates were at least 2-fold more resistant to SP-A-mediated killing by human macrophages than their respective early isolates. We postulate that the reduction of OprH expression is a previously unrecognized adaptation of P. aeruginosa to the lung of individuals with CF that facilitates the escape of the microorganism from SP-A-mediated phagocytic killing.