Effective dose of a microbial inoculant is one to four cells in the rhizosphere

A single-cell approach for studying the growth potential and the establishment of bacteria in the barley phytosphere is presented, using Pseudomonas fluorescens strain with the capability for biological control. The incidence of growth of one to four bacterial cells dispersed to the young rhizosphere approximated to 100%, and specific growth rate averaged 0.05. Net growth occurred for cells added to the rhizosphere at densities between 1 and 100,000 cells, while at higher densities population sizes declined, but always approached 10(5)-10(6) cells per rhizosphere. No net growth was observed in bulk soil, and cells died in the phyllosphere. Our results showed that bacterial establishment was more related to the availability of microhabitats supporting growth, than related to the number of bacteria released.