How does phosphate trigger pseudomonas aeruginosa infections?

By changing genes: This is not completely understood, but there is experimental evidence from animal experiments and such, that the pseudomonas grown in a low phosphate environment will respond on changes in what genes the germ activates. These changes lead the bacteria to make chemicals that increase its ability to grow and cause damage. In patients with low phosphate levels this may mean > risk of infection.

Inducible virulence: Pseudomonas demonstrates increased virulence when it is grown in an environment that starves it of phosphate; it affects expression of virulence genes when the organism detects low levels of phosphate.