Siderophore production by phytopathogenic microbial species

The production of siderophores (microbial iron transport agents) and siderophore-like substances was examined in the gram-negative bacterial plant pathogens, Agro-bacterium tumefaciens, Erwinia carotovora carotovora, Pseudomonas syringae pv. phaseolicola, and Erwinia amylovora, and in the saprophytic bacterium Agrobacterium radiobacter. This attribute was found to be widespread among the organisms surveyed. All but 2 of 27 different strains of A. tumefaciens and A. radiobacter investigated produced catechol-containing compounds during low-iron culture. One of these non-cat-echol-producing strains formed no detectable siderophore, and the other produced hydroxamate-like substances. The compound 2,3-dihydroxybenzoic acid as well as two water-soluble, ninhydrin-reacting catechols were found in iron-deficient cultures of E. c. carotovora 78. These phenolates enhanced the growth of the organism when stressed for iron. In response to iron-limiting growth, P. s. phaseolicola G50 produced substances which reacted with iron to give one purple and two orange-colored compounds. A mixture of these ferric complexes had growth factor activity for the organism when it was cultured in an iron-poor medium. Neither catechol nor hydroxamate-type siderophores were produced by E. amylovora Ea178 or Ea213. However, siderophore activity was detected in the supernatant fluids of low-iron cultures of both strains.