USE OF AN EXOTIC CARBON SOURCE TO SELECTIVELY INCREASE METABOLIC-ACTIVITY AND GROWTH OF PSEUDOMONAS-PUTIDA IN SOIL

Respiration and growth of Pseudomonas putida PpG7, containing cataboli c plasmid NAH7, was determined in three agricultural field soils amend ed with the carbon source salicylate. The addition of salicylate to so il significantly increased the population of PpG7. However, there was a lack of relationship between microbial numbers and activity as deter mined by evolution of CO2. In soils containing 30 to 1,500 mug of sali cylate per g, metabolic activities of PpG7 peaked between 18 and 42 h and population densities increased approximately 10(1)- to 10(5)-fold. However, the metabolic activity of PpG7 rapidly declined after salicy late was utilized, whereas peak population densities were maintained f or the duration of the experiments (5 to 7 days). Thus, elevated popul ation densities of PpG7 were represented by inactive cells. Soil type had only minor effects on respiration rates or growth curves of PpG7 w hen amended with comparable concentrations of salicylate. Respiration and growth rates were optimal at concentrations between 300 and 1,000 mug of salicylate per g in the test soils. At 1,500 to 2,500 mug/g, re spiration and growth of PpG7 were initially suppressed, but after a sh ort lag time both attained levels similar to or greater than those res ulting from the use of lower concentrations of salicylate. The culturi ng of PpG7 on a salicylate-amended medium to induce salicylate-degrada tive enzymes did not affect the lag time before utilization of salicyl ate in soil. Although PpG7 competed well with fungi for the substrate, suppression of fungal populations with cycloheximide resulted in sign ificantly increased population densities of PpG7 in two of three soils amended with salicylate. The beneficial activities of bacteria in soi l are discussed in relation to population density, population metaboli c activity, and selective carbon source utilization.