Growth interactions of Arthrobacter globiformis and Pseudomonas sp. in relation to the rhizosphere effect.

Abstract

Studies on the influence of plant roots on the soil microflora have shown that the number of Gram-negative rods in the rhizosphere is relatively higher than the number of Gram-positive rods, coccoid rods, and sporeforming types. This ecological phenomenon was duplicated and studied in a model system using a Pseudomonas sp. as representative of the rhizosphere and Arthrobacter globiformis as representative of the soil flora. Growth of A. globiformis was strongly suppressed in the presence of the pseudomonad in root extracts of mature plants and in a medium containing casamino acids, yeast extract, glucose, and mineral salts (CAYG medium); suppression was less marked in soil extract. The pseudomonad was unaffected in the association. A. globiformis was inhibited in the first 48-hour incubation in CAYG medium by acid elaborated by the pseudomonad; the pH dropped to 5.3 in 16 hours but rose steadily to alkaline conditions after 48 hours, resulting in a delayed increase in the number of A. globiformis to approximately that of a pure culture, in 5 days. Under cultural conditions favoring pigment production by the pseudomonad, growth of A. globiformis was completely inhibited throughout this period. Another toxic principle was also produced by the pseudomonad. This substance was biologically active in mixed culture against the Arthrobacter but was present in low concentration.