For pathogenic bacteria, the ability to acquire
iron is vital to survival in the host. In consequence, many genes involved in
iron acquisition are associated with bacterial virulence. Pseudomonas fluorescens is a bacterial pathogen to a variety of farmed fish. However, the global regulatory function of
iron in pathogenic P. fluorescens is essentially unknown. In this study, in order to identify
proteins affected by
iron condition at the expression level, we performed proteomic analysis to compare the global
protein profiles of P. fluorescens strain TSS, a fish pathogen, cultured under
iron-replete and
iron-deplete conditions. Twenty-two differentially expressed
proteins were identified, most of which were confirmed to be regulated by
iron at the
mRNA level. To investigate their potential involvement in virulence, the genes encoding four of the 22
proteins, i.e. HemO (
heme oxygenase), PspB (
serine protease), Sod (
superoxide dismutase), and TfeR (TonB-dependent outermembrane
ferric enterobactin receptor), were knocked out, and the pathogenicity of the mutants was examined in a model of turbot (Scophthalmus maximus). The results showed that compared to the wild type, the hemO, pspB, and tfeR knockouts were significantly impaired in the ability to survive in host serum, to invade host tissues, and to cause host mortality. Immunization of turbot with recombinant TfeR (rTfeR) and PspB induced production of specific serum
antibodies and significant protections against lethal TSS challenge. Further analysis showed that rTfeR
antibodies recognized and bound to TSS, and that treatment of TSS with rTfeR
antibodies significantly impaired the infectivity of TSS to fish cells. Taken together, these results indicate for the first time that in pathogenic P. fluorescens,
iron affects the expression of a large number of
proteins including those that are involved in host
infection.