Purpose: The aim of this work was to characterize a putative siderophore locus of P. fragi and determine whether strains that possess this region secrete siderophores.
Methods: A putative siderophore locus was identified in 12 P. fragi isolates of meat, milk, and non-food origin. Protein homology and domain analysis of locus components was performed, as well as a search for protein binding sites for regulatory proteins. Siderophore production of the strains was tested using Chrome azurol S (CAS) agar and supernatant assays.
Results: A genomic region of approximately 10 kb identified in 12 P. fragi strains was found to be homologous to the xss operon in xanthomonads, which encodes genes for xanthoferrin (α-hydroxycarboxylate-type siderophore) and xanthoferrin-mediated iron uptake, contributing to growth and virulence of these plant pathogens under iron-limiting conditions. Like the xss operon, this region in P. fragi comprised seven genes: Five siderophore biosynthesis genes, a TonB-dependent siderophore receptor gene and a membrane spanning transporter gene with homology to multidrug efflux pumps of the major facilitator superfamily. Upstream of this region, a potential Fur binding site was identified, which contained 13/19 residues conserved in the Xanthomonas campestris pv. campestris Fur-box consensus sequence. Moreover, siderophore production of the strains was confirmed using both CAS agar and supernatant assays.
Significance: Characterization of this locus and the role these genes play in iron metabolism may help us understand how P. fragi can compete so efficiently with other meat and milk contaminants and may lead to the development of effective strategies to limit P. fragi growth and spoilage of food.