The production of phenazine antibiotics by the Pseudomonas aureofaciens strain with plasmid-controlled resistance to cobalt and nickel

Plasmid pBS501 responsible for the resistance of the wild-type Pseudomonas sp. BS501 (pBS501) to cobalt and nickel ions was conjugatively transferred to the rhizosphere Pseudomonas aureofaciens strain BS1393, which is able to synthesize phenazine antibiotics and to suppress a wide range of phytopathogenic microorganisms. The transconjugant P. aureofaciens BS1393 (pBS501) turned out to be resistant to cobalt and nickel with an MIC of 8 mM. When grown in a synthetic medium with 0.25 mM cobalt, the transconjugant accumulated 6 times more cobalt than the wild-type strain BS501 (pBS501) (1.2 and 0.2 microgram Co/mg protein). Electron microscopic studies showed that cobalt accumulates on the surface of transconjugant cells in the form of electron-opaque granules. In a culture medium with 2 mM cobalt or nickel, strain BS1393 produced phenazine-1-carboxylic acid in trace amounts. The transconjugant P. aureofaciens BS1393 (pBS501) produced this antibiotic in still smaller amounts. Unlike the parent strain BS1393, the transconjugant P. aureofaciens BS1393 (pBS501) was able to suppress in vitro the growth of the phytopathogenic fungus Gaeumannomyces graminis var. tritici 1818 in a medium containing 0.5 mM cobalt or nickel.