Production of antibiotics by Pseudomonas cepacia as an agent for biological control of soilborne plant pathogens

Antibiotics produced by Pseudomonas cepacia strain RB425 obtained from a lettuce root and active as a biological control agent against several soilborne plant pathogens, were studied to determine the mechanism of disease suppression by this strain. Cultures of RB425 were extracted with chloroform or ethyl acetate and the antibiotics isolated in the pure or semi-pure form by silica gel column chromatography, gel filtration and high performance liquid chromatography. The active fractions were detected by bioautography on TLC plates using Rhizoctonia solani and Venicillium dahliae as sensitive indicators of antibiotic activity following each fractionation. On the basis of u.V., i.r., NMR and mass spectra, the antibiotics were shown to be similar or identical to pyrrolnitrin and two kinds of pseudane, 2-(2-heptenyl)-3-methyl-4-quinolinol and 2-(2-nonenyl)-3-methyl-4-quinoIinol. The antibiotics showed high activity against several species of fungal pathogens including Pyricularia oryzae as well as R. solani and V. dahliae, but relatively less antibacterial activity. These substances may be involved in disease suppression by P. cepacia RB425.