In vitro interactions between Pseudomonas strains. Bacterial suspensions were made by suspending cells cultured for 24 h at 28°C on KB-agar plates in sterile 10 mM MgSO4. Densities of the bacterial suspensions were determined spectrophotometrically at 660 nm. The bacterial strains were adjusted to 107 CFU/ml and two droplets of 5 µl were spotted on KB-agar plates, each approximately 1.5 cm from the edge. To determine the influence of iron-regulated metabolites, such as pseudobactins, the same experiment was performed on KB agar supplemented with 200 µM FeCl3. The spot-inoculated plates were incubated at 28°C for 48 h. Subsequently, a suspension of the target strain (107 CFU/ml) was atomized over the spot-inoculated plates, and zones of growth inhibition of the target strain around the spot-inoculated strains were measured after an additional incubation period of 24 h at 28°C.