Elevated c-di-GMP Levels and Expression of the Type III Secretion System Promote Corneal Infection by Pseudomonas aeruginosa.

Pseudomonas aeruginosa is generally believed to establish biofilm-associated infections under the regulation of the secondary messenger c-di-GMP. To evaluate P. aeruginosa biofilm physiology during ocular infections, comparative transcriptomic analysis was performed on wild-type P. aeruginosa PAO1, a Δ mutant strain (high c-di-GMP levels), and a p--containing strain (low c-di-GMP levels) from mouse corneal infection, as well as biofilm and planktonic cultures. The c-di-GMP content in P. aeruginosa during corneal infection was monitored using a fluorescent c-di-GMP reporter strain. Biofilm-related genes were induced in PAO1 compared to planktonic bacteria. Several diguanylate cyclases and phosphodiesterases were commonly regulated in PAO1 and biofilm compared to planktonic bacteria. Several exopolysaccharide genes and motility genes were induced and downregulated, respectively, in PAO1 and the Δ mutant compared to the p-containing strain. Elevation of c-di-GMP levels in P. aeruginosa began as early as 2 h postinfection. The Δ mutant was less susceptible to host clearance than the p-containing strain and could suppress host immune responses. The type III secretion system (T3SS) was induced in PAO1 compared to biofilm bacteria. A Δ mutant with a defective T3SS was more susceptible to host clearance than a Δ mutant with a functional T3SS. Our study suggests that elevated intracellular c-di-GMP levels and T3SS activity in P. aeruginosa are necessary for establishment of infection and modulation of host immune responses in mouse cornea.