Bacterial strains.

The recipient strain for all plasmid constructions was E. coli DH5αF′IQ (Invitrogen). For strain construction, plasmids were transferred into P. aeruginosa via conjugation utilizing E. coli SM10 λpir. Bacterial cultures were grown in lysogeny broth (LB) or on LB plates containing 15 g/liter agar. For P. aeruginosa strains, gentamicin (Gent; 30 μg/ml), carbenicillin (200 μg/ml), or tetracycline (Tet; 35 μg/ml) was used for selection when required. For P. aeruginosa, cells were grown in liquid LB cultures containing 10 mM MgCl₂ and 0.5 mM CaCl₂ (LB-MC). Where indicated, type III secretion was triggered by removing calcium from the medium by the addition of EGTA (5 mM final concentration; LB-MC-EGTA). A list of strains and plasmids can be found in Table 1.

Strains and plasmids used in this study