The role of bacterial motility in the survival and spread of Pseudomonas fluorescens in soil and in the attachment and colonisation of wheat roots

Motile and non-motile strains of Pseudomonas fluorescens SBW25 were constru cted using different combinations of the lacZY, xylE and aph marker genes w hich allowed their detection and differentiation in soil, root and seed sam ples. The survival of motile and non-motile strains was investigated in bot h non-competitive and competitive assays in water and non-sterile soil. Alt hough there was no difference between strains in water, the motile strain s urvived in significantly greater numbers than the non-motile strain after 2 1 days in soil. There was no significant difference between competitive ass ays, where motile and non-motile cells were co-inoculated into soil, and no ncompetitive assays where strains were inoculated separately. Bacterial sur vival decreased as matric potential increased from -224 to -17 kPa but matr ic potential had no significant effect on motile compared to non-motile str ains. Vertical spread of both motile and nonmotile strains was detected 6.4 mm from the inoculum zone after 14 days in the absence of percolating wate r. There was no significant difference, for either strain, in distance move d from the inoculum zone after 14, 26 or 40 days. The motile strain had a s ignificant advantage in attachment to sterile wheat roots in both non-compe titive and competitive studies. When the spatial colonisation of wheal root systems was assessed in non-sterile soil, there was no significant differe nce between the motile and non-motile strain from either seed or soil inocu lum. However, when the whole root system was assessed as one sample unit, d ifferences could be detected. Bacterial motility could contribute to surviv al in soil and the initial phase of colonisation, where attachment and move ment onto the root surface are important. (C) 2001 Federation of European M icrobiological Societies. Published by Elsevier Science B.V. All rights res erved.