Growth and survival of bacteria introduced into carbon-amended soil

Agrobacterium tumefaciens, Pseudomonas sp. and Corynebacterium sp. grew in glucose-amended soil, but their populations subsequently declined. Protozoa multiplied as the numbers of cells of these species fell. The rise and subsequent fall in densities of the three bacterial species did not correlate with changes in the populations of lytic or antibiotic-producing organisms or in the lytic activity of the soil. Myxobacteria, bacteriophages and Bdellovibrio were not detected in the soil. The densities of the three bacterial species and indigenous bacteria were far greater and the subsequent falls in cell numbers were less marked in glucose-amended soil treated with cycloheximide and nystatin than in soil not receiving the two cucaryotic inhibitors. Addition of the two inhibitors caused a decline in the numbers of trophozoitcs and a rise in the numbers of protozoan cysts. The three bacterial species grew extensively after inoculation into sterile soil supplemented with glucose, but their population sizes were appreciably smaller and the numbers subsequently declined if the sterile soil also was inoculated with a mixture of prutozoa. The results suggest that predation by protozoa is a major factor limiting the growth and causing the decline of bacterial populations introduced into soil amended with readily-available organic nutrients.