Wave-like development of bacterial communities and a biocontrol agent introduced in the wheat rhizosphere

For successful application of biocontrol agents of root pathogens we need to know the ecology of the agent and the native microbial community. We studied in detail the spatial patterns of the bacterial community in the rhizosphere of wheat and, in separate experiments, the distribution along wheat roots of Pseudomonas fluorescens 32 gfp introduced into soil. Rhizospere and bulk soil samples were collected every 1.5 cm along wheat roots, amounting to 25 or more pairs of samples per root. CFU of copiotrophic (CB) and oligotrophic bacteria (OB) were counted on rich and poor agar media. Microbial communities were analyzed by DGGE on 16S rDNA directly isolated from soil and from mixed colonies. CFU of CB, OB, and gfp P. fluorescens, and microscopic counts of gfp cells exhibited significant wave-like oscillations, revealed by harmonic analysis. Distributions of DGGE fragments suggested a succession of amplicons along the root. The most intense amplicons fluctuated in phase with CFU harmonics; less intense ones fluctuated out of phase. This suggests that individual species like P. fluorescens fluctuate in tandem with the oscillations observed for total CFUs, and probably run counter to infections by some plant pathogens as shown earlier