Bacterial control of Agromyces ramosus in soil

Abstract

Agromyces ramosus occurs in high numbers in many soils. It also is a known predator of various gram-positive and gram-negative soil bacteria, including Azotobacter vinelandii. Based on this, it would seem that, in natural soil, A. ramosus should control the population sizes of these soil bacteria. As a partial test of this assumption, we examined the possibility that soil might contain other bacterial predators that could hold A. ramosus in check. Three gram-negative bacterial predators of A. ramosus were isolated from soil. When one of these predators, strain N-1, was added to natural soil, it exhibited an attack – counter attack phenomenon in its interactions with A. ramosus. The indigenous A. ramosus cells in soil, or added A. ramosus cells, produced mycelium that approached, then lysed, approximately one-third of the N-1 cells. The surviving N-1 cells, however, then proceeded to lyse the A. ramosus mycelium, but not the rod-form cells that had fragmented from the mycelium. Strain N-1 then multiplied. This sequence also occurred if Azotobacter vinelandii was added with A. ramosus to soil, either with or without addition of N-1 cells. N-1 attacked the A. ramosus mycelium that was attacking Azotobacter vinelandii. In soil and with pure cultures in the laboratory, the dormant rod-form cells of A. ramosus that fragmented from the mycelium were not attacked. A growth initiation factor seemed to be involved in the attack – counter attack relationship of N-1 and A. ramosus. Strain N-1 and the other two gram-negative predators mentioned above could attack a variety of bacterial species in soil, in addition to A. ramosus which in itself is a predator. Thus, some sort of hierarchy of bacterial predation seems to exist in soil.