Survival and growth of bacteria introduced into soil

A study was conducted of the survival and multiplication of bacteria introduced into soil. In a loam, the populations of Rhizobium leguminosarum biovar phaseoti, R. meliloti, Agrobacterium tumefaciens, Micrococcus flavus, Corynebacterium sp., Pseudomonas sp. 2K and Pseudomonas sp. 1G declined by one to two orders of magnitude. Some of these species initially increased in numbers in this soil upon the addition of 0.1 or 1.0% glucose, but others did not proliferate. However, the populations of all species subsequently declined. The patterns of decline of A. tumefaciens, Pseudomonas sp. B4, Streptococcus faecalis and M.flavus in silt loam were similar. Because the patterns of decline of none of the test species were similar in unamended sterile soil or, except for M. flavus and S. faecalis, in sterile buffer, the reductions in population sizes were not a result of toxins already present in the soil or starvation, but were related to the activities of other microorganisms. The population size of an asporogenous strain of Bacillus subtilis declined markedly in nonsterile and sterile soil and in buffer, and it fell to a small number in nonsterile soil amended with 0.1% glucose. Because cellular slime molds, bacteriophages, bdellovibrios and myxobacteria were not found, they are not responsible for the reduction in numbers of the test species. Protozoa increased in number as the populations of the test species fell. The extents of decline of A. tumefaciens,Pseudomonas sp. 2K, Corynebacterium sp. and M. flavus were diminished if glucose-amended or unamended soil was treated with cycloheximide, a eucaryotic inhibitor. Antibiotic-producing and lytic microorganisms able to destroy the test species were present, their numbers varying with the test organism. It is suggested that the populations of bacteria introduced into soil may be reduced because of their susceptibility to predation, starvation or possibly antibiotic-producing or lytic microorganisms.