Identification of Pseudomonas fluorescens strains isolated from raw pork and chicken that produce siderophores antagonistic towards foodborne pathogens

Three strains of Pseudomonas fluorescens (strains 13, 43, and 52) isolated from ground pork or chicken meat exhibited broad-spectrum antimicrobial activity against both gram-positive (Listeria monocytogenes and Staphylococcus aureus) and gram-negative (Salmonella enteritidis, Escherichia coli O157:H7, Yersinia enterocolitica, and Campylobacter jejuni) foodborne pathogens, but not lactic acid bacteria (Lactobacillus and Pediococcus). Inhibition was not due to production of organic acids or other antimicrobials, but was related to the availability of iron, with the addition of ferric chloride to agar media reducing the inhibitory effect of all three Pseudomonas strains against indicator bacteria. All three Pseudomonas strains exhibited an orange zone on chrome azurol sulfonate agar plates, and crude culture supernatant fluids had a maximum absorbance at 400 nm. Collectively, these data strongly indicate the involvement of a siderophore(s) in the antimicrobial activity of P. fluorescens 13, 43, and 52.