Characterization of iron uptake systems in Campylobacter jejuni

Abstract

Acquisition of iron contributes to bacterial pathogenesis. In C. jejuni, there are more than five systems which contribute to ferric iron uptake, mainly consisting of siderophore receptor systems. In vivo, C. jejuni encounters the anaerobic environment of the intestine where the predominant state of iron is likely the ferrous ion. A homolog of FeoB, a high-affinity ferrous iron transporter, is encoded in the C. jejuni genome. Initial experiments with C. jejuni demonstrated low levels of ⁵⁵Fe²⁺ uptake. Furthermore, mutations in feoB failed to inhibit ferrous iron uptake when compared to the parental isolate. No differences were noted between the wild type and mutant strains in epithelial cell invasion, macrophage survival, or peroxide sensitivity. The feo locus also failed to complement an E. coli ΔfeoB mutant. Sequence analysis indicated various mutations in C. jejuni strains. Notably, strains RM1221 and F38011 contained frameshift mutations that caused early termination of the protein. M129 had an amino acid substitution in a critical region of the protein involved in nucleotide binding. While the presence of magnesium significantly inhibited⁵⁵Fe²⁺ uptake (> 3-fold), inhibition of CorA activity did not affect uptake of ⁵⁵Fe²⁺. Viability of C. jejuni required the function of CorA and mutants at this locus were lethal. When compared to wild-type, a putative fhuA mutant was attenuated in epithelial cell invasion (∼1000-fold) and intra-macrophage survival (∼10-fold) but not in a piglet model of the disease. The putative mutant failed to produce a protein with similar size to FhuA and an immunoblot reacted with a protein of the size of FhuA in the parent but not in the mutant. In summary, ferrous iron uptake is not mediated by FeoB or CorA in C. jejuni. Homologs of feoB in different strains of C. jejuni appear to have acquired various mutations resulting in non-functional proteins. The corA locus is absolutely required for growth of C. jejuni since it is likely the only magnesium transporter in this species. The FhuABD system is required for the optimal invasion of epithelial cells and intra-macrophage survival of C. jejuni. However, mutants in fhuABD do not attenuate the organism in vivo.