To test whether laboratory passage of our P. syringae 1448a strain might have resulted in inactivation of the yersiniabactin genes by phase-shifting or another reversible mechanism, we repeatedly sub-cultured the pvd-/acr- double mutant in iron-limiting KB broth on a daily basis for

7 days, each day plating out a dilution that gave ca. 103 colonies on CAS agar. Duplicate FK506 cell line plates were incubated at either 22°C or 28°C for up to 72 h, but no siderophore-secreting colonies were recovered. We therefore concluded that P. syringae 1448a produces only two high-affinity siderophores in response to iron deprivation, pyoverdine and achromobactin. When each of the WT, pvd-, acr-, and pvd-/acr- strains were grown in liquid media and subjected to a modified CAS assay that we developed to measure iron acquisition by factors secreted into the culture supernatant, the results were consistent with the phenotypes check details observed for each strain on CAS agar (Figure 5). These results confirmed that P. syringae 1448a is able to employ achromobactin as a temperature-regulated secondary siderophore that is secreted into the extracellular environment for active uptake of iron; but also suggested that the presence

of pyoverdine is able to mask any phenotypic effects due to achromobactin alone. Figure 5 Liquid CAS assay. 96-well plate wells containing 200 μl unamended King’s B liquid media

were inoculated in triplicate from synchronized overnight cultures of the following strains: WT (black squares), acr- (white circles), pvd- (grey circles), and pvd-/acr- (grey diamonds). A triplicate media-only control (black triangles) was also included. Plates were incubated with shaking at either 22°C (A) or 28°C (B) for 48 h. Cells were then pelleted and 150 μl supernatant removed to fresh wells. CAS dye (30 μl) was added to each well and the rate at which iron was removed from the dye by secreted factors in the supernatant was followed at OD 655 (monitoring loss of blue coloration). Error bars are presented as ± 1 standard deviation. Assessment of Selleck JSH-23 relative fitness of mutant strains under iron starvation conditions To more precisely quantify the contribution of each siderophore Ureohydrolase under varying degrees of iron starvation, a serial dilution experiment was performed, employing EDDHA concentrations diluted 1:2 from 800 μg/ml down to 0.2 μg/ml in KB media in a 96-well plate. The WT, pvd-, acr-, and pvd-/acr- strains were replica-inoculated into each well and incubated with shaking at 22°C for 24 h, following which culture turbidity was measured. IC50 values (indicating the concentration of EDDHA that yielded only 50% turbidity relative to the unchallenged control) were calculated for each of the strains using Sigma Plot.