Effect of carbon source supply and its location on competition between inoculated and established bacterial strains in sterile soil microcosm

Abstract

The aim of this work was to study how the location of a carbon source and of bacterial cells in soil can enhance the growth of a bacterial inoculum. Studies were performed using irradiated soil and two pairs of strains of Bradyrhizobium japonicum and Pseudomonas fluorescens. For each species, an antibiotic‐resistant mutant was used as inoculant and introduced into a sterile soil pre‐colonised with its parent strain. The inocula and a glycerol amendment were introduced together or separately into soil using porous microgranules or by spraying a suspension onto the bulk soil. Competition was assessed by plate counts of both inoculant and established populations. Both B. japonicum and P. fluorescens parent strains established at a high density level (109 cells g−1 of dry soil) in the sterile soil. The effects of the amendment on the inoculant strain in soil were strongly dependent on the way the cells and the substrate were introduced into the soil. Joint introduction of cells and substrate into soil improved the growth of the inoculant strain, but the effect of the amendment was also shown to be dependent on the inoculant carrier. Location of glycerol and the inoculant strain in microgranules gave a competitive advantage to the inoculated bacterium for a part of the amendment. We suggest that diffusion of glycerol from microgranules to the soil limits the competitive advantage of such location effects.

Impact of substrates and cell immobilization on siderophore activity by Pseudomonads in a Fe and/or Cr, Hg, Pb containing-medium

“…Indeed diffusion coefficients of glucose, fructose, sucrose and lactose in Ca-alginate membrane liquid-core capsules and in Ca-alginate beads immobilizing microorganisms were shown to be only 5-20% lower than the corresponding diffusivity in water [52,53] which explained why the concentration of solute in the bulk liquid medium reached equilibrium after a few minutes. The same conclusion was drawn by Duquenne et al [29] who suggested that diffusion of glycerol from microgranules to the soil limited the competitive advantage of such location effect. Conversely, skim milk remained in beads due to its chemical properties involving interactions with alginate.…”

“…In this aim, immobilization of viable cells was shown by several authors as a convenient technique for improving their survival [13][14][15]. Also the location of substrate into the immobilization carrier can give a competitive advantage to the inoculated bacterium as already shown with glycerol and P. fluorescens immobilized cells in microgranules [29]. Likewise, we demonstrated in a previous work [11] the positive effect of cell immobilization in the maintenance of the siderophore activity in Fe-containing media.…”

Evaluation of shelf life and rock phosphate solubilization of Burkholderia sp. in nutrient-amended clay, rice bran and rock phosphate-based granular formulation

“…1c). Added glycerol might have diffused out of the carrier which would decrease the amount of substrate that could be used by the inoculant strain for its growth (Duquenne et al 1999). Significantly higher numbers of viable cells were enumerated from clay and rice bran + RP amended with 1% glucose than 0.1% (Fig.1e, f).…”

Siderophore production by using free and immobilized cells of two pseudomonads cultivated in a medium enriched with Fe and/or toxic metals (Cr, Hg, Pb)

“…However, an appropriate formulation of microbial inocula requires further investigation. Indeed survival and consequently siderophore production can most likely be enhanced thanks to the supply of carbon substrates in the immobilization matrix (Duquenne et al, 1999), which could at the same time selectively promote immobilized cell growth and not indigenous microbial population in soil. Comparison of the siderophore production by free and immobilized cells of P. aeruginosa and P. fluorescens in the minimal medium by measuring the specific production of immobilized cells to their free counterpart ratio.…”